I'm reading this thread at ARFCOM about some mall ninja pervert who stalks dirtbike trails, and they link to this YouTube video where some 13 year olds in a race run his sick ass over in the middle of the woods because he's buried himself in leaves.
I honestly have no advice to give on this one. Unless you have your Little Johnny competing in races with a cut down Winchester 1887 strapped to the side of his bike, there's nothing you can really do to totally prepare your youngster to defend against all the sick shit that mankind can come up with. You can only cover so many scenarios.
***ETA: From the discussion at this forum, the guy has been seen all over the place during dirt bike races and such. He carries the ninja outfit with him into the woods in a backpack, and dons it before burrying himself in leaves or dirt. Whenever anyone confronts him, he doesn't speak and quickly walks off. Weird.
Here's the video: