T1Oracle: It's trivial to get a new e-mail address at places like hotmail, or gmail, or yahoo. Enough people use those services that you can't just ban creation from those addresses. Also, if you run your own mail server, you can create an arbitrary number of e-mail addresses.

As someone kindly mentioned, the best route would be to charge. Even if you were offering a free RPG, im sure most people wouldent mind a $1 startup charge (no montly). Wont really make you any money, but it will cut many of the 9-13 year olds that are oft the ones banned. Have them explain to mommy they need a new user again, when they read its a one time fee.