It depends on what they say or do. If they are snarky, usually something obnoxious slips out of my mouth before I realize what I am saying. It's an automatic reflex. But, if they have done something really terrible, I hire a lawyer and sue them. Okay, I lie. I walk away and completely shut them out.
People never treat me badly. They may say something out of ignorance or stupidity, but I just let them bask in it and ignore it. I may insult them back with a word they don't know so they don't know they have been insulted.