You need another tree stand set up on the first tree stand, and just blunt them. You need some human intervention to be sure you aren't just shocking or leg holding some kid who went up for a look. Something that works great on bikes is loosening the front wheel, they steal the bike, or at least they ride off on it, and their face ends up on the ground, can't think of a stand equivalent. You need to rig a stand with one of those thigns they put on motorcycles that actually track the thief to their home.

A more passive approach would be to get rid of the ladder stand and use some simmons steps and some kind of easy hanging tree stand. Whatever the simplest system you could handle hanging every time. Another possibility would be to weld up your own stands and build in your name or "stolen", or some such.