Set up a dummy blind where they can easily see it and let them fool around there while you quietly hunt another place. Frankly, if they are setting up on their own property or on property they have permission to hunt, there isn't much you can do about their stand placement as long as it isn't on your land. It may be rude, but it ain't illegal.
As to the trespassing, if I knew where they were driving the ATVs onto the property, I'd be thinking about burying a couple boards with nails driven through them on your side of the property line. 4 flats may make them think about not doing it again. If they moved a corner post, go to the county courthouse and get the legal description of the property boundaries and be sure you're right. If you are, move the post back where it should be and make sure it is made of metal and set in concrete. Then take a photo of it to show it is in the legal place. if they move it again, call the law. It goes without saying if your land isn't posted, do so right now!