Get your game cams and check them. Check the picture numbers to make sure they're not tampered with.

Also, start moving the game cams and start doubling/tripling them over each other... leaving one obvious and one way above, hidden in the trees. (disable the flash on the hidden one so they dont find it)

If he's messing with your hunting, I'd bet he's messing with your property. he obviously doesnt have much in the way of morals.

Start carrying a camera(video and/or still digital). Post signs at the property boundary that they are on serveillance cameras( i think you have to post a sign about it in most states if you're going to try to take pictures of it)....

Contact a local hunting organization for a good landowners legal representative (lawyer), if not your landowners lawyer.

If you're like me, you prolly aint got $300/hr layin around, but it would be worth it to me to nail the jerk to a legal tree.

If you can't tell, I've got a bit of a vindictive streak...... I can think of much more interesting things to do, but you said keep it legal.... (not to mention ethical)...

Good luck and keep us posted