Have you tried “Bumface”? 
Or you could just point and laugh.
I was walking passed some youngsters one day, texting someone on my mobile, the youngsters started yelling insults at me. I stopped looked at them and said “What? What did you say? Was that supposed to be funny or intelligent or something?” then walked away. One of them apologised to me the next time he saw me, and the others maintain a casual silence when I pass them now … 
One time some kids started effing and blinding as myself and my pal were passing (obviously trying to look hard), I stopped and stared at them and said [dead loud], “god I wish I was big and tough enough to stand on a street corner and swear when someone walks passed” My pal was so mortified she ran off …
In response to “EEUUUUUUUAAAA”., try:
“That’s what happens when you use a cactus as a suppository!”
Him: EEEEUUUUAUUAAAAAUAUUAUAU!!!
You: Put some spit on your thumb and it might not hurt as bad.
Francesca
What stratagy did you finally settle on?
“Did any of your parents’ kids live?”
Or just blow them a kiss.
“Don’t make me come down to where you work and knock the dick out of your mouth.”
I really think George Carlin did it best.
Doesn’t get much better than that.