Completely ignore them
OR
Say something that doesn't make sense or is very strange (I've thrown a few people off doing this and it's probably quite disturbed them judging by their faces)
OR
Pretend you recognise them and make some familiar comment like "Hey! See you on Saturday then".
Having said this, I'm female so I'm not likely to turn around and try to fight a guy or do anything other than run off.
Have had some crappy incidents - things on fire thrown at me and my friends, money thrown, eggs thrown, drink cans thrown, obscene comments from lorry drivers (when running on my own, was particularly scary), and just the general stupid comments. Had someone scream at me really loudly from a car window at some traffic lights when it was pitch black and I was running on my own - that scared the shit out of me too.
This isn't just confined to the USA - I live in England. I think though, if you respond like you're mental most people will leave you alone.