Poisoning the Well

Poisoning the well is the use of a preemptive abusive or circumstantial ad hominem attack against an opponent with the purpose of discrediting or ridiculing everything they are about to say. It generally has the following form:

1. Unfavorable information (true or false) about person A is presented.

2. Therefore, (explicitly or implicitly) any claims about to be made by person A should be dismissed. 

Examples:

"Don't listen to anything Steve may tell you, he's a socialist."

or 

"Before you listen to my opponent, may I remind you that he has been to prison."

Practically speaking, poisoning the well is a form of ad hominem, and as such, one should follow the guidelines of analyzing an ad hominem to determine if it is being used in a fallacious manner. This essentially means questioning the relevancy of the attack on the claims presented by the person for whom the attack was directed against. 

Wikipedia: Poisoning the Well

Logicallyfallacious.com: Poisoning the Well