Well, what it really comes down to is that most of the people who left either had been wanting to quit for a while and a clear made it easier for them or they were asswipes. If you'll notice, I still have about the same number of people playing as I did before, with many praising the new EXP system (and acknowledged that some don't care for it, but play anyway).
It's really hard to control anyone because of how tribes is setup. In an MMORPG, everybody has an account and that account is easily banned without having to resort to an IP ban. Here, if you ban a jerk they're probably using a dynamic IP in the same range as five innocent players. What you can try though, is just delete/repassword/clear their characters on a constant basis. If they're playing to play, they'll probably eventually get sick of it and leave.
Also, having some admins to help out is always nice. *points to UG, Pecker, and the rest*
1,000,000 seconds of jail can get annoying really fast, too