I was at this other board, and I noticed they had a great idea. They had a "warning" count. Moderators could give each person a warning if they violated a rule, and 5 warnings got their account banned.
I rather like the idea :) I think it'd work well here.
Just my 2 cents - It gets annoying when the same people are consistently breaking rules and generally being jerks. You'd figure getting a few warnings would give them enough of a reason to shape up.