A video camera! And don't be afraid that you are harming this kid buy turning him into the police. He has a better chance of turning into an upstanding citizen one day if you turn him in now instead of letting him get away with it. In the past, I've let stuff slide and even talked to parents only to see nothing done and those same kids end up in jail or trouble later in their life. Put a stop to it now while they are still a juvenile.