I would venture to guess that most of the time, when someone yells something to you while you're running, they are just trying to get a rise out of you.
If you respond to them with something stupid (like most of the dumb responses in this thread already), you're doing exactly what they want. In their mind, they got the best of you because they managed to piss you off to the point where you come back with a lame response and make yourself look stupid.
The best way to deal with it is to not say anything. Put yourself in their shoes for a minute. If you were yelling out stuff to someone to get a rise out of them and they didn't respond or look in your direction whatsoever, you'd probably be a little pissed. More than likely, the person yelling out the dumb comments to the runner would think to themselves..."Damn, they didn't even hear me..." OR "Damn, they probably thought that I was yelling at someone else."
Either way, you're not giving them the benefit of showing your emotions.
Now, if however, you think of a good comeback "a mile down the road" and just so happen to bump into these d-bags later on in the day (i.e. when you're not running), by all means...go ahead and let them have it. They'll be caught off guard and think to themselves..."Shit, that really came back to bite me in the ass." If you don't see them again...then so be it. You still got the best of them.