I've heard that some cities have designed graffiti tracking applications for smartphones. People can take a picture of the graffiti/vandalism, geotag it, and submit it via the app. I think this would really help cut down - if vandals know that reporting is going to be ten times more efficient, they might think twice about doing it.
There is also a service called Graffiti Tracker,(graffititracker.net) that issues their clients GPS enabled cameras, that city officials can use to photograph graffiti. Then it's uploaded to a website where it's analyzed for commonalities, and potential gang references.