It's sounds to me like your dad has inadvertantly given this guy permission. He knows the guy hunts on his property, yet doesn't care and allows it. The only way you're going to prevent this guy from hunting from your stand is to either tell him, take the stand down, or move the stand to another location no as easy to find. What I would do is put some sort of cable or chain across the seat so no one would be able to sit in it. Then you can keep the key. If you find the cable or chain cut, then it would be confrontation time.