Dude I think you are fighting a lost cause unless you can actually catch them in the act. I dont think that you can keep a dirt bike or a fourwheeler out of anything. Unless somebody knows something that I dont. You can try spikes but they will grow accustomed to knowing where they are, I did. However I do know that the property owners inside our small town city limits used to string up barb wire about chest high and/or cables to keep fourwheelers out, I first hand experienced some of those in my younger days. You would be driving your normal little trail just out ridding and all of a sudden. There youd be either thrown over then handle bars because your brakes actually caught or flat backed with the fourwheeler rollin down the trail. I dont suggest that method however. You never know what could happen. In the present day and age you could probably get sued for stuff like that if there was an "accident."