The best ways to confront an invalidator may include simply ignoring them, proving their accusations to be false, or to respond to their negativity with kindness or humor.
Bob: "I believe you are mistaken, sir."
Invalditor: "Who cares? Go away. No one likes you here. You're a scumbag!"
Bob: "Is there a problem?"
Invalidator: "You're an asshole. We all know it. Get out of here. You're not worth it."
Bob: "I think your attitude is completely rude, sir."
Invalidator: "I'm not being rude. I'm just telling it to you like it is. No one else has the heart to tell you this, but you're a useless asshole. Nothing you say matters. You're not likeable. You're not a person. You are nobody. Know this. Believe it harder than anything else."
Bob: "Whatever you say."