Depends on various things.
I would fight back, and even go to the extent to make sure he gets a beat down that he will remember for the rest of his life ( to make sure no future problems arise ). That is if I know the person.
I've learned my lesson not to fightback against people you don't know, I just leave it be. Plus who cares what some random thinks of your anyways, whether you're a wussy or can tear him into two.
I personally don't instiagte, not any more at least.
Alcohol, a.k.a liquid courage, has a big influence on the above two statements.
Know your enemy