I've been in a similar situation before.. There was a new guy at our track, on a YZ125. No numbers, just lots of stickers and baggy pants. He rode the beginner practice, and was giving my friend's daughter a hard time on her TTR125L. He would cut her off, ride way closer than she was comfortable with (on purpose) and he wiped her out once, then stopped and roosted her.
I went out and stuffed him big time in the first corner I saw him in, took his front wheel out and knocked him over the berm. That was that.
He kept doing the same antics later in the day, so I went out again but he'd get a glimpse of me coming so he'd stop on the side of the track.
I was just giving him a taste of his own medicine, he didn't like it much. He didn't seem to learn anything from it though.
Haven't seen him there since then. I felt a little bad about it after, but I figure he deserved it.:mad: