How about a mechanism such that, if you get a certain number or percentage of down-votes in a particular time period (a day or a week), your ability to post is blocked for a set period of time?

I suppose the person could post anonymously, but I wouldn't mind seeing that go away anyway.

I don't know the answer, but I do know that after taking a couple years off from PM (for unrelated reasons), I was surprised to return and see this person still posting -- and posting the same bad answers as two years ago, in some cases.