Unless there is a noise ordinance it doesn't sound good. Are there any streams of any size that they ride through? If so the DEP and CORPS can make life miserable for a long time. There may be county or some sort of state water management laws also. As to the riding on other people's property, on the boundary in their trails take sawed off metal fence post (or other similar metal pieces, maybe rebar) and drive them about 4 inches above the ground and try to sharpen them a little, it won't take much with those soft 4 wheeler tires. Then cover them with some leaves.