There is a graffiti removal maching that uses a suction like perspex glass that clamps onto the wall or area to be cleand. You then use a scriber type object to remove the affected area. I test drove one so to speak 2 years ago, its a long cylindrical type contraption. Takes a fair bit of practice. uses sand at high pressure to score of the graffiti. Forget what it was called, I could find out, maybe it went by the way side, it was a bit hair brained.