I don't think it would work too well, real jerks would just find other posts and assign antipoints to those. That, and would that only protect the thread starter, not participants or subsequent posts by the same person?
Here's one to try tossing out: After a certain time, threads start to become 'stale', and as they do so, any points given within the thread count for less and less until the thread is officially 'stale' and points can no longer be assigned to posts within it...
