I would do what the cops do in Jersey City when you are illegally parked, they put the boot on the tire and a notice to call them on the windshield.

I would lock it up with chain and lock. Than place a note that states you are trasspassing and if you would like your tree stand unlocked, please call (place your numer here). Take a picture also. This way you are not taking what is not yours, only making it difficult for him to retreive it.

This way he would think twice about hunting there again.

Tomster