Perhaps you’ve noticed a burst of graffiti vandalism around the neighborhood lately. Hazel gives some very sound advice on dealing with it on the mailing list:
Be sure to call the graffiti hotline, especially if it is on public property. The number is 206-684-7587 and they are very responsive. The faster you get it off the more effective. If it is public property, the city will arrange to have it removed. If it is on private property, you should also take a photo of it and send to the police department and the city.