I used to gun hunt a fairly out of the way place in NE Michigan. I sat with my back against the same stump for several years, each year I would brush in a minimal ground blind. I killed a deer there every year. One year, the day before opening day, I went out to brush up my spot. When I got there, there was a guy building a blind around "my" stump. I told him thats where I usually spent opening day, and he said he had been "hunting that spot for years". Liar. Oh well. What to do, shoot him? I left and found a different spot. I saw him 3 days later. He was skunked, I had a nice forhorn hanging. Karma is a real thing.
If I had a treestand hanging on public land and found somebody in it, I would tell them it's mine. If they offered to get down, I would probably decline, as the place was buggered up anyway. Whether they were polite or not, I would wait them out a short distance away, and when they started to climb down I would come back and remove my property. I don't have much desire to hunt in a spot so easily found anyway.
Hunting for me is a peaceful moment in a hectic life. I don't bring bad-ass type aggression into the woods. Also, I carry concealed at all times, and part of that responsibility is to avoid conflict.
As for folks who will steal a stand, whether on public or private property, they are a thief. It's a shame we can't just shoot them. Wouldn't it be great if a concealed carry permit came with coupons?