You state that it was a spot that you never sat before and you put salt out when you left for the summer. What was that 3 months ago?
Perhaps the salt was all used up when he put out his bait and camera and had know idea it had been there or that you planned to return months later. Maybe this was a spot he has hunted before.
If he is breaking the law with the stand a 4 wheeler thats one thing but don't assume that spot was all yours for the taking.
That being said he could be a jagoff and I feel your pain with having to hunt public land.