Poisoning the well
Poisoning the well is a logical fallacy in which the person making the fallacy seeks to discredit an opponent before that opponent has presented any arguments. It is an argumentum ad hominem and a red herring argument.
The negative information presented to discredit the opponent may be either true or false, but, as it is known to be negative, is presented solely to persuade the audience to view the opponent's later arguments as false regardless of their rationale.
- "When John E. Jones decided in 2005 to 'traipse into' the controversial area of evolution and science education, deciding the scientific merit of intelligent design as a federal court judge in Dover, PA, he may have only dreamed of the day when he would see himself on the silver screen. [...] Now PBS and NOVA are teaming up to produce what may be Judge Jones' dream come true. 'Judgment Day: Intelligent Design on Trial' is a special 2-hour program devoted to the Dover trial as Judge Jones saw it." -- Anika Jones, "What NOVA Won't Tell You about Dover"
- "Scientists at Los Alamos — the good folks who unleashed the terror of atomic bombs on the world — are now proposing that we use bees to sniff out bomb-toting terrorists." (fictional example)
Presuppositional apologists, such as Sye Ten Bruggencate, are fond of poisoning the well.  He dismisses his opponent's arguments because his opponent cannot rule out being a brain in a vat (emphasis added):
- "Matt says that truth is that which comports with reality. He admits he doesn't know what is ultimately real, therefore according to his world view, he can't know anything to be true and has zero basis for challenging my claim that it is reasonable to believe that God exists. [...] That's all we heard... from someone who doesn't know he is a brain in a vat."
This argument is a red herring because a person can challenge an argument even if they are a brain in a vat.
The quality of an argument does not depend on the person making the argument, or that person's character, or any unsavory qualities the person might possess. The argument should stand on its own.
However, people can easily be biased against anything a "bad" person says. Poisoning the well exploits this propensity.