This is the downside of not owning property unfortunately.

You can do everything the right way, and people can still set up and hunt right beside you if they want. If they have permission from the owner, they have as much right as you do to be on the property.

If you value the hunting spot, then I suggest taking steps to try to secure it for your future. Talk to the landowner and see what agreement you can find. Maybe it's you trading work or some other good for exclusive access to that piece of ground. Maybe it's a lease situation etc.
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what they did isn't right. But that you already knew that and there really isn't much you can do about it unfortunately.

-Jake