This letter turned up earlier this week. It clarifies who is responsible for the defaced buildings on Lilyfield Road, and notes that they have been cleaned up before. It's nice to see that they do care.
The letter states that the previous cleanups have been quite expensive, which I can understand. But that's one reason why I think it needs to be stamped on quickly as soon as it appears - it is cheap to paint over one tag, but expensive to paint over dozens of them. Property owners that get onto graffiti quickly probably face lower costs in the long term than those that let it slide.
Here endeth the lesson.