I like Terr's idea. If someone reaches say 2000 AP's, just put the "no more need" popup. Maybe even put a big purple bar there or something, so people know that they are no longer able to assign antipoints. You could possible still agree/disagree with the post, but the antipoints wouldn't be counted.

You could also implement this so that people could "opt out" of the antipoint system. If you have a big black bar, that means that the user doesn't wish to receive antipoints. Again, you could still rate the post, but the antipoints wouldn't count.