

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.
There different reasons for wanting privacy or advocating for for personal data privacy and implimenting strategies to support that.
If ever you are in trouble with the state or make the wrong person on the internet angry, the more you’ve publically shared the easier it will be for them to find something to use against you. So… you can try not to get on anybody’s bad side to limit that possibility or you can do your best to compartmentalize your online activities.
It should be possible to ask for help or show vuneralbility online without telling anybody (or thing) absolutely everything about you every time.