I learned that panhandlers, at least in Chicago, interpret polite negative responses as opportunity and will continue to bother me. So, I have taken to saying flatly, "Not happening." It's not so rude or demeaning that I inadvertently pick a fight, but it's blunt enough to let them know that they're just wasting their time with me.
When I'm in a place where these folks will follow you around, I turn as robotic as possible and answer once (or maybe twice to make things perfectly clear) with a simple "no". I try to leave no room for interpreting playfulness or agressiveness.
