If this were the sort of game where you really needed a lot more time to invest in each character, then banning them *might* discourage them, but really you can make a new character in this and fuck around immediately, so it's tough to figure out how to fix it.
In the beginning we had you get email verification for each account which *maybe* helped a bit, but these days it is incredibly easy to just make throwaway email accounts.
Seriously, probably the only thing that would help with this sort of thing is putting a price tag on each new account, and nobody wants that.
The thing that really amazes me the most is how much time all these people seem to have to devote to something like fucking with an old, pretty obscure browser game. I mean, wow.