That's a huge problem around where I live, Cobleskill. The private landowners are about ready to close everything off. I'm afraid the yahoos deserve it, too. Trails that for 25 years of my own use were a couple of feet wide are now 6-8 feet wide and muddy most of the time.
The bad part of it is that people like me and some of my long time neighbors have been walking or riding horses on these trails for decades with no unhappiness from the landowners, but in just a few short years the ATV/dirt bikes have shat in the nest and messed it up for everyone. I sure hope when the fences and gates go up foot traffic will still be allowed.
IMO, ATV's make a great tool and a really sucky toy.