Yeah, the charity "HI THERE!" <dances sideways in front of you> can piss right off.
It's harder with guys asking for money on the street. There's a a guy not that far from my first flat in Edinburgh that's still there now, every day, and allegedly has a flat and does very well out of it (as I'm sure a lot of the guys on the touristy streets do well over what I earn in a day). I was in the pub underneath that flat once, and he was in there buying fags out of the fag machine, which pissed me off, because I smoked at the time and would never buy them out of a machine at twice the retail price. There was a 24 hour shop over the road that sold them at retail price.
Equally, at the same kind of time, I gave a fiver to a guy on a really crappy cold rainy night out on Gorgie Road that was sitting in a doorway, miles from the tourist spots and he was chuffed to bits and went into the takeaway to get some food. That was great, and there was no way he was scamming anyone.