Dismisssing the other person by claiming their motive is nafarious.

I think this seems to be a common trope by calling women "TERFs" for criticizing gender identity acts that could potentially allow someone to speak their way into a women's prison, or calling people " racist " for criticism of religious practices such as the burqa, or even affirmative action.
It's basically " You are "bad thing" therefore I can dismiss you, please shut up" . I'd say it's slightly different to  just saying " you're an idiot therefore I can dismiss you please shut up ".
I'd assume it's a form of Ad hominem, but is it a specific kind of ad hominem.

Dismisssing the other person by claiming their motive is [nefarious].

This by itself is ad hominem (circumstantial), but your example seems to be a bit different:

I think this seems to be a common trope by calling women "TERFs" for criticizing gender identity acts that could potentially allow someone to speak their way into a women's prison, or calling people " racist " for criticism of religious practices such as the burqa, or even affirmative action.

That could be considered poisoning the well. The person is asserted to be a bigot because they gave an opinion on something. The suggestion is that what the person wants to say should not be considered, or is unreasonable, because of that.

Bear in mind though, although this tactic is typically used to shut down debate, if accusations of bigotry can be backed up, the other person may have gained a point. Real prejudice and bigotry, after all, are still very real.

I think this kind of responding to an argument falls under the identity fallacy . And although I don't find many differences from the identity fallacy, there are also these fallacies that are in Dr. Bo's book
genetic fallacy ,
psychogenetic fallacy ,
bulverism ,
ad hominem (circumstantial)