If this is your only spot to hunt, then I wouldn't feel bad about asking them to remove their stuff. At least talk to them. Maybe they have other places to hunt as well.

But, if you other land to hunt on and don't really have to hunt there, then I'd be a somewhat lenient on them and ask them to remove it at the end of the season.

If it's not your's yet, then it's the landowners call. Maybe they were not suppose to be ther in the first place.