If I'm to be honest my initial reaction to being unfairly honked at is to lash out, not physically (as this is not in my nature or capability) but to somehow annoy the honker in retaliation. This is bad - and is the common human failing that drives forum flame wars. However - here is the best thing to do when someone honks or abuses you in some way ... nothing. Do nothing.

Not a thing. Try not to flinch, keep riding, same pace, same direction, don't look at them, certainly don't shout or make gestures - just keep calmly going as you were. They *hate* this complete disempowerment - it's infuriating. Normally, as a driver, when you lean on the horn it makes something happen outside - but somehow this magic power is all broken now.

I used to pretend that I had mistaken them for a friend - and give a happy wave of recognition. This kind of worked, but it's so easily misinterpreted as giving them the finger (which would mean that you're annoyed - and that something has happened). "Nothing" is far more powerful - you're treating them like that yappy little dog that runs back and forth behind the front yard fence. It is permissible to quietly mutter to yourself "yeah, yeah, woof bloody woof"