Poisoning the well
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The usual method is to point out the unpleasant nature of the person making the opposing argument, in which case it is a special case of a personal attack or ad hominem. In general, "to poison the well" means to pre-provide any information that could produce a biased opinion of the reasoning, positive or negative.
It can be done subtly or quite blatantly. A subtle way of poisoning the well would be to use particular adjectives in introducing something that would influence people who are about to hear an argument. In a more blatant display, someone can make an outright personal attack in an introduction. For example, asking people to remember that a person has been in prison before listening to their statements; the well is now "poisoned" because people are likely to distrust a person making an argument knowing that they're a convict, regardless of the reasoning that they put forward.