The graffiti "artists" caught at it should do public service cleaning it up, whether their own or not. The victim gets his notice and a perp is assigned, no muss, no fuss.
Presumably their cleanup crews consist of graffiti offenders who are put on this detail for community service.
They're saying that you'll be fined if you don't notify the city to clean it or if you don't clean it yourself.
basically, they're saying that one has no right to voluntarily have graffiti on their house.