i know all to well what you are facing man. my dad has some land and kids are alway riding on it and this ahole who thinks he owns it. they tore up a feeder i had out there and i never put one back. this season i decided to put a trail camera up and to keep them from taking it i took some 1/2 iron and fired up the lincoln. i made a strong box with a chain around the tree they cant steal it now.
but about your problem heres what you do like mentioned in the other posts get some lag bolts and screw them in next bring a file or battery grinder and strip the head of the bolt now it impossible to remove. also try hanging a sign letting them know that they have beem photographed, even if you dont have a camera set up its a simple detterant. good luck