Graffiti may be art to the people that do it but if you're doing it on someone else's property, it's nothing more than vandalism. I can't stand advocates of it. If you do it and try to condone it, I propose this: allow me to do it to the side of your house.
With that out of the way, I do condone this:
Reverse graffiti.
I also say get out there and do it. Not only is it great art, it forces the community to clean up the mess
they made, not a mess
you made.