the wonderful pixelator

The pixelatior is a cool anti advertising tool that people can put on advertising screens so that they just show bright light and cool colors. You can't arrest the people who use them because they are not breaking any laws. I thought that this was really cool but I don't think I will be able to remember to do this next time I go to the big city. Still it sounds like a cool idea. About the graffiti that people put on walls that clean them up; I wonder if you could arrest them if the stuff on the walls was offensive enough, or would that violate the 1st amendment right?