I have noticed that occasionally someone will deface their own question, presumably in an attempt to cover their tracks. I have also noticed that when someone does this the question attracts plenty of downvotes. Now, the SE system promotes questions with higher votes and quietly hides those with a negative score. Therefore, downvoting such a question hides the it from view. Which is what the OP wants!
My point is this:
It makes no sense to down-vote a question which the OP has defaced. This is aiding and abetting them. You should rollback/flag the question and not downvote it. If you are really annoyed at the OP and want to annoy them in return, upvote.
What think you?