Alternative word to describe 'pigeon-holing' someone to win an argument [duplicate]

This sounds like a combination of logical fallacies. Mostly an ad hominem attack (attacking the person, not their argument), but also a touch of red herring (distracting with facts/claims irrelevant to the original topic).

Argumentum ad hominem (argument directed at the person). This is the error of attacking the character or motives of a person who has stated an idea, rather than the idea itself. The most obvious example of this fallacy is when one debater maligns the character of another debater (e.g, "The members of the opposition are a couple of fascists!"), but this is actually not that common. A more typical manifestation of argumentum ad hominem is attacking a source of information -- for example, responding to a quotation from Richard Nixon on the subject of free trade with China by saying, "We all know Nixon was a liar and a cheat, so why should we believe anything he says?" Argumentum ad hominem also occurs when someone's arguments are discounted merely because they stand to benefit from the policy they advocate -- such as Bill Gates arguing against antitrust, rich people arguing for lower taxes, white people arguing against affirmative action, minorities arguing for affirmative action, etc. In all of these cases, the relevant question is not who makes the argument, but whether the argument is valid.

Red herring. This means exactly what you think it means: introducing irrelevant facts or arguments to distract from the question at hand. For example, "The opposition claims that welfare dependency leads to higher crime rates -- but how are poor people supposed to keep a roof over their heads without our help?" It is perfectly valid to ask this question as part of the broader debate, but to pose it as a response to the argument about welfare leading to crime is fallacious.

[Logical Fallacies and the Art of Debate - CSUN]

Terms like "ad hominem" and "straw man" (attacking the person rather than his argument, or switching to an argument you can easily win) don't exactly describe what's going on in OP's example context.

If someone says Since you voted for Trump, you're a misogynistic racist whose opinions are worthless, what they're doing is stereotyping...

stereotype (noun)
a set idea that people have about what someone or something is like, especially an idea that is wrong

(Where typecast and pigeonhole are effectively synonyms for the derived verb sense.)