If ***it's been up for over a year*** then it's abandoned. Likely the person forgot where he set it up.

Disclaimer I'm not a lawyer. But I don't see any way that could be harassing a hunter. No one has been hunting for a year. He's only storing the tree stand on your land.

Again, see disclaimer. But if someone leaves something on my property then it's mine. I don't see any criminal case being made that I stole it. I don't see any civil case either but talk to an actual lawyer if you are worried about that.