It's been my experience that if you say something to the kids about it, they'll graduate to something worse. They'll eventually stop if they see that it doesn't bother you (even though it clearly does). I've had far worse things happen to the point where I, like Donna, freaked out one day, charged right into the kid's house where his mother was sitting in the kitchen drinking a coke, I grabbed him by his shirt, pinned him up against the wall and said some things that I shouldn't have. (No, no foul language, but I spelled out exactly what would happen to him if he came on my property again for any reason.) I don't know who was more shocked, the kid, his mother, my son or me. Try to let it go.