There is something to be said about "I don't owe you shit, and I'm not dignifying you with a response" being a valid and even respectable answer. I think most of us would give that back to a lot of Lefties asking us for "proof" of something.

But that's also a fine line where you have to actually be innocent to begin with, and not actually respond beyond brushing it off flippantly. They've failed both of those so it simply looks worse for them now to do so.