Limiting 1 account to 1 user is the holy grail of PBBGs (persistant browser based games) developers. Unfortunately, a fully satisfactory way to do this was never found. Nowadays, developers try to design their games to work with multiple accounts.
In the old days, it is common for PBBGs to impose a "no multi" rule. They would try to catch "multis" - users with multiple accounts. This might be done through a "1 account per ip address" rule or painstakingly analyzing user behaviors. People are still able to get away with multi-ing despite these measures. In your case, you still cannot stop people from creating accounts from different computers. I can make one at home, one at my workplace...which happens to be an university, so I can make one in the library too. Or homes with multiple computers can use multiple IP addresses to multi.
One solution I can think of in your case is :
- Start players with $0.
- Make them do some work for their starting capital.
- This work is avaliable forever.
- The returns from doing this work is far below all other activities.
The last point ensures that once they obtain enough starting capital, their time is better spent doing other things. Under this system, whatever they can do with new multi accounts can be done on their main account anyway, but their main account have access to even more profitable ventures.