Poison the Well

Poisoning the Well is an argumentum ad hominem in which an opponent is criticized before his argument has been heard. It is fallacy of relevance, since it doesn't address the argument (being only a guess at it) but the arguer.

Bruce Thompson:

...it is not so much aimed at persuading the audience that the opponent's view is false (which would be an argument) as it is aimed at getting the audience to refuse to listen to the opponent's argument in the first place.

Fallacy Files:

The phrase "poisoning the well" ultimately alludes to the medieval European myth that the black plague was caused by Jews poisoning town wells—a myth which was used as an excuse to persecute Jews.