Dang, already? Sounds like Arc is working out well.

Still, though, any statstical approach (which is what I assume you're using) is going to have some lag because you have to wait for a statistically-significant sample. You can catch any given offender after he's caused an O(1) amount of trouble, but if the offenders don't correlate with each other, you can get overwhelmed by a sufficiently large number of them. Spammers certainly do correlate with each other, but it's not obvious that voting rings do.