Studies have shown that when graffiti is removed, it shows up again soon after. But, when it keeps being removed, the perpetrators eventually give up. I'm not saying you should remove all graffiti everywhere but if you pick one or two spots and keep cleaning them up, eventually they will remain graffiti free.
And, don't think you're alone in this. A lot of cities and merchants are happy to pay for clean up supplies and paint if they're getting free labour. So why not get a group together...maybe your boy scout troupe or maybe some big and little brothers and get crackin'.
A great lesson for the kids. And you'll feel better for it!