> therefore an important relief valve

Until it is done to your small business or home, then it is no longer an "important relief valve". The solution to reducing graffiti is multi-part. Here are a few ideas: (1) Pass a state law to restrict the sale of spray paint -- you need a special license to buy it. (2) Pass a local law to reward citizens who provide evidence of taggers (video, photos, etc.). If the city can convict, you are rewarded. Make the reward large enough (1000+ USD?) to be strongly encouraging. (3) Create public spaces where people are allowed to spay paint. This is a little bit like skate parks.