Cocky people who hang around in groups on street corners sneering at random people in public and call them names and try to provoke people into a response and yet seem surprised when you actually stand up to them
The funniest thing is to not even look at them and just give them the finger.
Trust me, I can think of a few times I've done it, it's hilarious. A couple of chavs will be making monkey noises at you or whatever their incest-formed brain believes is a magnificently witty insult and as soon as you flip them off there come cries of "BRUV HE WEREN'T DOIN NUFFIN" "MAN DAS OUT OF ORDER" etc.
One time I was sitting on a bench with my friend and that happened, they were sitting on a bench a bit further along, I gave them the finger and even after sitting there
for the next hour they didn't even have the balls to come over and say anything more than telling me that I had been incredibly rude. You say that they act surprised when you react, but honestly I don't think that their shrivelled brains are actually intelligent to comprehend the idea of a response in the first place. They're just that stupid.
EDIT:
I hate you and your music taste.
I'm suddenly reminded of the fond days of our music related debates. Ah, banter.