If you have nothing to give them to help out, (or you don't want to), you just have to learn to tune them out. It's no different than being constantly hounded by vendors in Mexico trying to sell you everything under the sun. I tell them (Mexican vendors) no thanks and walk on by.
I don't think you gotta be a dick to them (the homeless) or an asshole. Don't create unnecessary confrontation. Just tell them "sorry, no change today buddy." and walk on by. They are never going to stop asking... they have nothing else to do.
Be polite dude. A guy who already has nothing to lose probably wouldn't hesitate much to kill your dog or fuck you up if you're essentially asking for it. But be prepared to lay the smack down if they get aggressive. If one of them assaults you, a couple of well placed strikes can easily snap a collarbone or knee, fracture a pelvis so he can't stand, crush a trachea or shut down a kidney. Move quick and use enough force to put him out of commission. Then go call the cops so he can get medical attention.
