Poisoning the well

Discredit the other person before they speak. Or discredit the topic or argument that they may support.

There are many ways of discrediting the person. Call them names. Talk about their lies. Show them to be unworthy. Tell how they are unintelligent, crazy or otherwise undesirable, inferior and not worth listening to, let alone believing.

To discredit the topic or argument, indicate how it is patently absurd, proven to be false or that only fools would support it.

Examples:

1. Well, Jane will tell you something else, but then she always lived on the other side of the tracks.
2. Mike doesn't have a degree, but he does speak nicely, doesn't he.
3. Only an idiot would consider Didactus to have any useful opinion.
4. Everybody knows that cold fusion is a proven impossibility. Jack: did you have something to say on this.

By discrediting the other person, you are also effectively discrediting anything they say by reducing their authority. If the other person is there, a public attack forces them onto the defensive, socially obliging them to respond first to the attack and hence distracting them from their main argument. If the other person is not there, then they cannot defend themselves.

Personal attack always has its hazards, and other people, especially rescuers, may well leap to their defense.