Here in Zurich there's a city office where you can report graffiti that's on city property and they eventually will fix it but they have a big backlog. I learned though that if it's obscene graffiti they move it to the top of the list - there was a typical FCZ (football club Zurich) on a wall near where I live for months then one night someone spray-painted a big penis over it and the graffiti was fixed in just a couple of days later...