I've learned to respond to bums the same way they approach me. That tends to reinforce the "treat others as you wish to be treated" rule to the ones who aren't suffering some grave mental defect.

There's this guy who panhandles at the light where northbound I75 service drive traffic crosses the freeway towards westbound 8mile. He's an old vet who holds up a cardboard sign, he's always asking for work, and he smiles and waves at you even when you roll up your window and give him dirty looks. 2 winters ago, I pull up behind a car stopped at that light and saw a guy wearing a tie, driving a white Benz roll down his widow and throw a drink on him. It was about 25 degrees outside.

You don't have to hug the bums or give them your money. But for godsakes at least have the decency to politely ignore them if nothing else. They are some of the most vulnerable people in our society, whether or not they choose to be there.