Comebacks, I suck at them. I'm just dumbstruck. So here are a couple I prepared earlier:
The new 'talk to the hand'
'What I'm hearing is this' (a hand talking)
'What I want to hear is this' (a hand not talking)
I'm not sure if this will work in reality, but it would be funny to just yell 'NOTHING' back, if you've got nothing. Sometimes just doing something weird diffuses situations. I got beat up once, I used to think back to it, and the way I thought I could have stopped it was to go in for a kiss with the guy on the ground instead of fighting back.
This isn't really a comeback, rather a way of not having to talk to someone when you run into them. Mouth, 'I've got laryngitis. I can't talk right now.'