That's a possibility, but consider the amount of time it would take to do that. Someone with that much of a vendetta and that much spare time probably knows better ways to harass someone than by voting down posts on PM.

I think self-regulation is, as usual, a pretty good solution. I regularly hit Worst Nodes and bump up stuff that doesn't deserve more than -1 or 0. There's three or four posts of that kind every day (depending on your criteria), so higher-level monks interested in keeping the peace would do well to spend a few votes there.