Get metal stakes (re-bar, etc.) and surround the tree. Wrap heavy duty fencing around the stakes. Put far enough from the tree that they can't nibble through.
Seriously those rascals are hard to get a shot on. We fight them every year on a creek that makes one of the boundaries to our property. They work odd hours and are hard to pattern. Patience and a good hide is the only way we can get a shot. Night time is your friend if you have the gear to do it.