Word for dismissing someone's opinions as racist, liberal, etc, instead of debating back

I'm looking for either a single word or phrase that would describe either someone or the action of dismissing someone's opinions as something "socially unpopular", without giving any reasoning why, either just to spite them, because they have a poor counter-argument, have been backed into a corner, or something similar.

For example, someone who supports something politically incorrect (like, say, stronger gun control or help for male domestic abuse) might have their views dismissed as racist or sexist or liberal without actually acknowledging and debating against them, thus labeling the former as something they might not be.

I was talking to a friend earlier about politics, and she complained about Germany's immigration problem. She said one issue was that many politicians that try to support stronger immigration laws would immediately be shot down as racist or a nazi and forced to resign, instead of being presented with counter-arguments. Regardless of whether you agree with them or not, what would describe that?

EDIT: Forgot to specify -- the word or phrase I'm looking for would be used in an informal context. And to address further questions, to be used when the person's opinion has legitimate worth and isn't just "being racist" etc, i.e. the dismisser is incorrect, and is only making these accusations because they can't argue further. (Added emphasis to the last sentence of the first paragraph)

It seems you want a word for someone refusing to engage in discussion, because they regard the subject as not open to debate. To dismiss out of hand is one option which has already been suggested, or more neutrally you might have to disengage from discussion.

In a similar vein I would suggest shut down debate.

I also detail how accusations of trolling were used as a justification for shutting down debates about community expectations ... — Bergstrom, Kelly. "'Don’t feed the troll': Shutting down debate about community expectations on Reddit.com". First Monday, Volume 16, Number 8.

In this quote, it is alleged that users refused to engage in discussion about community expectations and instead just "called troll", which is similar to your example where people "call out racism/sexism" rather than engaging.

I helped shut down an abortion debate between two men because my uterus isn't up for their discussion. — Title of an article by Niamh Mcintyre. The Independent, 18 November 2014.

Here the writer states that she prevented, rather than engaged in, a debate, because she felt the participants lacked relevant standing.

If you want give a more specific reason for the disengagement, rather than just acknowledge that the it occurred, you might like play the race card, or equivalently play the sexism card.

Playing the race card is an idiomatic phrase that refers to exploitation of either racist or anti-racist attitudes by accusing others of racism. — Wikipedia entry for "Race card", as of 3 July 2015.

An example usage from the British press:

Disgraced former mayor Lutfur Rahman (pictured) is said to have played the race card to silence opponents - and his deputy today reiterated claims there is deep seated racism in the borough of Tower Hamlets, in east London. — "Vote rigging party is STILL playing the racism card: Day after mayor is forced out, deputy blames 'Islamophobia'". Daily Mail, 25 April 2015.

And one from the States:

Democrats won't be able to play the sexism card if Republicans pit former Hewlett-Packard CEO Carly Fiorina against Democrat Hillary Clinton, says John Sununu ... — "John Sununu: Carly vs. Hillary Would End Dems' Phony Cries of Sexism". Newsmax, 31 March 2015

Both of these news sources are notoriously conservative: I don't think either card-playing idiom is very likely to be used by a political liberal or centrist except ironically! So these are less neutral terms than "shutting down debate" is.

Alice: "We should have stronger immigration laws"

Bob: "You're racist. Why would anyone listen to a racist like her, folks?"

This response doesn't take into account the arguments made by Alice. Bob is attacking her character. This is a case of ad hominem.

An ad hominem (Latin for "to the man" or "to the person"), short for argumentum ad hominem, means responding to arguments by attacking a person's character, rather than to the content of their arguments. When used inappropriately, it is a fallacy in which a claim or argument is dismissed on the basis of some irrelevant fact or supposition about the author or the person being criticized.

PS: This is not 'poisoning the well'. You poison the well before someone drinks the water from it. If Charlie came along and told Bob not to listen to Alice (as she was in Hitler Youth or something), before Alice even said something, that would be poisoning the well.

I would say that the action is committing an Association Fallacy

is an inductive informal fallacy of the type hasty generalization or red herring which asserts that qualities of one thing are inherently qualities of another, merely by an irrelevant association. The two types are sometimes referred to as guilt by association and honor by association. Association fallacies are a special case of red herring, and can be based on an appeal to emotion.

(Wikipedia)

The association in the examples in the question would be that

• racists are anti-immigration
  therefore
• being anti-immigration is racist.

As for what to call someone who does that: I think it depends on their reasons - laziness, bigotry, deliberately antagonizing, etc. Perhaps an Association Fallacist but it's not a word.

It's a...

knee-jerk reaction - an immediate unthinking emotional reaction produced by an event or statement to which the reacting person is highly sensitive; - in persons with strong feelings on a topic, it may be very predictable. (from thefreedictionary.com)

I can't think of a single term that specifically also implies the person thus reacting assumes everyone else shares the same negative opinion of whatever is being summarily dismissed, so no "reasoned argument" is required. The best I could come up with would probably be something like That's just a groundless appeal to populism.