I recently came home to a bunch of kids vandalising my house mates Vespa (admittedly it had already been written off by the insurance but that's irrelavant). None of them were over the age of 14, and there must have been about 12 or 13 of them, boys and girls. I went ballistic, shouting and swearing, told them all to go home, which they did.
There hasn't been any more car/bike vandalism since.
Just put the fear of God into I say! A little respect goes a long way, I mean we were all kids once, weren't we?