As a bit of an aside:
Noted street artist Shepard Fairy was charged with felony vandalism over his graffiti artwork on abandoned buildings in Detroit, despite the fact that his artwork was attractive and a big improvement to the buildings. He has been charged with vandalism in other cities, as well. Art students in Ann Arbor graffiti-painted overpasses belonging to CSX railway and the police charged the students with vandalism.
In the Ann Arbor case, CSX stepped in to say they wanted to pay the students for the artwork already completed and hire them to paint other overpasses, so charges were dismissed. Shepard Fairy has not fared so well.
If the property owners object to the painting, it is vandalism - and anyone who looks to be involved can be arrested and potentially charged. I personally would not want to be standing too close to any vandal when someone calls the police.
If someone has an urge to spray paint, perhaps they might want to contact property owners and ask if they want their fences or garages painted. Not only will the "budding artists" avoid criminal charges, they can earn themselves some extra money.