I've never had anyone heckle me for riding fixed. I've been heckled for violating traffic laws, not wearing a helmet, riding hands-free, fixing a flat, etc etc etc.

Never for riding fixed, though.

I suppose if I were, my response would depend on the person heckling. If he was bigger than me, I'd just smile brightly and give a thumbs-up, confusing them. If I was bigger than the heckler, I'd tell them I had gears at one point, but I left all but one of them in his mom's a**.