same as when someone calls me anything in a mean way— quiet smoldering resentment and elaborate years-long revenge plots that end in their untimely death, from natural causes as far as the coroner can tell

Elbow to the jaw. The elbow is more suited for striking than the knuckles are.

But what are you?

                               /S

Find peace in yourself and fuck what anyone else thinks.

That's just kinky

Call them out on being an asshole if you can get away with it.

If the situation is less than safe, maybe introduce yourself with they/them instead of it/they/them. Not ideal but at least if there are bystanders around there's more chance that they'd feel okay calling out the person using "it" in a snarky way.

You could lightly stab them.