What if a game used a one-way string based on hardware similar to what Windows XP does to make each user unique... Would it be practical to implement this into a multiplayer game to say... make sure that banned people stay banned... Just a dumb question... maybe some people have some thoughts on this...

That''s what several games (including mine) already do. Of course it''s hackable but it will left most of the morons out of your game :-)