I learnt a long time ago to treat all humans with respect and dignity, some of those beggars are just simply out of luck by many circumstances beyond their control, if one responds with a simple sorry I do not have any spare change, and actually look at them eye to eye, they will respond to the fact that you have acknowledged them as being part of the human race, and not some kind of plague.
Even the winos respect you if you in responding respect them by answering in a simple human way, I think that SeƱor Moisheh is close to the actual fact, some one did something to elicit that kind of response.
Look at them eye to eye and tell them:
Lo siento mucho no tengo cambio.
or:
lo siento no tengo monedas.
Simple to the point and respectful on your part, the encounter is over.
A lot of beggars will thank you actually, you had the dignity to acknowledge them as humans, try it some time it is not that difficult.
navegator