Did you bop the SOB on the back of the head with your fist? Or better yet, if he is bending over, a swift kick to the right location will assure the tree-hugger will have no little saplings.
He is committing a crime, so you can stop him.
I sure would have, and then called the police. Police look at stealing plates as a pretty serious misdemeanor, since stolen plates are used on stolen vehicles.