Here's a grossly oversimplified view of the issue:
Generally trolls are better protected from harassment than non-trolls. An effective troll will consciously use the rules of the environment to make you feel miserable without giving you the opportunity to retaliate. That means that the more rules there are against trolling the more likely it will be that somebody finds a neat way to screw everyone over without allowing anyone to touch him.

A decade ago Ultima Online had a lot of trolls, but that was part of the system. Getting killed and robbed was something that happened a lot and that was to be expected. Much like in actual society the players that didn't want to be harassed put their heads together and developed strategies against the harassment and they succeeded at least partially because the game allowed them to defend themselves.

In short: One way (not at all the only one) to help peaceful players against aggressive players is by not interfering with artificial rules and letting them sort it out themselves. It works in real life.

Addendum: This doesn't work in all environments, especially in such where the caused harm is disproportional to the difficulty of the associated action.

You fail to prevent trolls because you think as developers not as trolls, your decisions only propagate this behavior and force players to become trolls.
Ask me who have trolled in the past plenty of times, and i believe all of my troll attempts are justified, i wont take any back.

How it starts ( the game forces you to become a troll):
a) friendly fire: if someone kills me in an online game. Example: wow (solution: disable).
b) kicks / downvotes: for no reason / unfairness/ or downvote just because they dont agree with me. (solution: disable).
c) report: threats that i will be reported for no reason or blaming me for something that is not true. (solution: disable)
d) money: only donors are allowed to chat, or have good gear. (solution: disable)
e) killsteal: (solution: disable)
e) chat: for instance example chat in league of legends : sucks - is op - is troll - is ks - afk - feeder - angry gamer saying "fu"
I think 100% of the trolling in league of legends happens due to this. Some nab ignorant "non-troll" starts like that forcing you to become a troll. (solution: disable???)

Measures :
1) Language filters: i will rephrase it so i am unaffected. (solution: disable)
2) Moderators: create a new account and byPass ip. Troll continues with new zeal for unjustified ban.(solution: disable)
3) Karma points: become a troll because they downvote for no reason.(solution: disable)
4) Rewarding players who help newbies (yes this is the only solution).
Build your game around helping players, exchanging gifts, Zynga is the only company that did this right (although i hate get XXX friends restrictions).
5) Vote kicking & abusing the vote kick(solution: disable)
6) Player ranking - (solution:unaffected: trolls are at all ranks).

Synopsis : i believe that trying to fight it aggressively with only create more trolls. The solutions are :
a) friendly sublimiar messages at loading screen.
b) game created at building good player relationships like zynga.

The best solution is public Read Identification. At least, a part of the community learn from World of Warcraft that majority of players want to stay anonymous so that they may hide from social stigma including trolling. Trolling just happens to be one of the social stigma that people want to hide from others.

Another point is that a community has a cap of 150 people. This is the major problem with online gaming in general. Players can only interact with this limited amount of tertiary level of social network. The human brain is not develop for larger network.

Getting rid of the global channels would get rid of these trolls. Of course, effort would have to be put into making better tools and mechanics. The only reason, I ever really needed the global chat channels, was when I was a new to the game. WoW made people want to pick flowers; You should be able to make helping new or people in general help desirable.

Another obvious method: Leave the game at LAN only, or invitation only via private games. Don't leave things so open and you risk less abuse. You wouldn't have to do something crazy and complicated, like storing statistics about a player to discern if he was acting decently or not (nor would you have to write the code that actually does this kind of stuff). You also wouldn't have to ban players, or deal with players complaining about being banned.

The obvious problem with this: How does a newbie get invited to play online if he doesn't have friends who also play this game to invite him? Simplest solution to that: Let him invite more. More specifically, let them invite each other over their social network of choice, so you're effectively out-sourcing the lobby to, for example, Facebook.

Another obvious problem: It does not allow for persistent worlds. Frankly, I've never been big on MMO's, but obviously an MMO wouldn't want to be invitation-only. That just defeats the purpose of that whole ever-complicated venture.

Removing chat, especially from a game like LoL, would not solve the issue. Trolling over chats are simply a symptom of underlying problems not the cause for them and in team based gameplay they are a fundamental part of communicating with other players. There are also many other ways to troll a player from within the game, most of which are on the whole a lot worse than a few angry words in a chat.

Global chats often server a social, trade or group forming purpose. Removing them may remove a large amount of the "spam trolls" (e.g. the [Dirge] spammers in WoW) but the truly serious trolling will still be done in private chats.

The idea of focusing on LAN or invite only games would be the solution for a fair few multiplayer games, but it seems the PUG options in multiplayer games are way to popular at the moment for developers to not include them.

This will be impossible if you ask me. There will always be more hardcore player yelling at new players then helping players.

You should turn it around, how would i make the game more fun for the noob player that gets yelled at.

Make a match making system that groups on skill and how much/often the guy plays. League of legends and SC2 has this but it's still crap, you often end up with a level 4 and a level 30 in your team.

Often the real flaming happens in very competitive games like the 2 i mentioned above. You can't really blame the flamers, hack i even rage a little sometimes when i get my 5th loss in a row becaus some new guys screw my fun, now both parties have no fun so match making is really important. You could let the player pick how broad the search can be, so that hardcore players will take more time to find a game but he won't get teamed with that lvl 5 or that higher level with a win rate of only 25%. The new player gets teamed with people that don't care all that much and might even help and give tips to the new player.

Karma point are a good idea too, only give bad karma points as good points can be abused by making multiple accounts and friends. When when a certain threshhold is reached monitor the player, a moderator could do that and give appropriate action.

Removing chat, especially from a game like LoL, would not solve the issue. Trolling over chats are simply a symptom of underlying problems not the cause for them and in team based gameplay they are a fundamental part of communicating with other players. There are also many other ways to troll a player from within the game, most of which are on the whole a lot worse than a few angry words in a chat.

Global chats often server a social, trade or group forming purpose. Removing them may remove a large amount of the "spam trolls" (e.g. the [Dirge] spammers in WoW) but the truly serious trolling will still be done in private chats.

The idea of focusing on LAN or invite only games would be the solution for a fair few multiplayer games, but it seems the PUG options in multiplayer games are way to popular at the moment for developers to not include them.

I have to ask, what do trolls do now? Global chat is the only place I see them. I occasionally, run into immature people, but I wouldn't call them trolls.

Well any chat can be used from /say to /yell to /party or even /whisper. It largely depends on the situation and the troll themselves.

The game play centred trolling can vary heavily from game to game. For instance in LoL you can shadow another team member making it close to impossible for them to play, in WoW one example is pulling a whole room of mobs and then leaving the group so they wipe. Some may view this is griefing and not trolling but they essentially mean the same thing.

People don't play well at all in general, and don't always have good manner. Rules in a game aren't going to change that. I wouldn't call that trolling.