I thought of this some time ago. Forums these days have some really nice ways to ban and distrupt trouble users, but it's not perfect.
Either these ideas:
1. Ban (or lock/suspend) user if they made x posts that are exactly the same (like one after another maybe).
2. Ban (or lock/suspend) user if they made x posts that have something in common (same images or certain words) one after another.
I've seen this work on image board scripts. Some tries a raid and they get banned even if there aren't staff online.
