Find some place... as an example, hexbear's instance has c/slop... to drop a link to the offending user's comment or post and whatever happens... happens.

You've got the option of Reporting the post/comment to the moderators. Which will require you knowing the moderation rules of the community in question to take advantage of.

Short bullet point/talking point responses. Replies with things like, "This is racist/sexist/genocide denial/etc", then disengage. You don't need to get into a long drawn out comment war/debat bro situation.

Make fun of them for being a goober.