There have always been, and will always be conflicts with illegal trail users in Westchester Parks. The best you can do is call the police when you encounter a motorized user in the park. I've personally seen moto and quad riders in Sprain- most are kids who could care less if what they're doing is right.
There was a real problem with one particular quad rider at Graham last year. He made the mistake of going out when there was a fresh couple inches of snow on the ground. One mountain biker followed his tracks back to his house, and had a nice conversation with the young guy's dad- problem solved, although there's still plenty of work that needs to be done to repair the damage.
Sprain really needs some summer growth to cover up all the off-trail riding that happened over the fall and winter. Although, that "playground" area really does have a ton of different lines, rocks, logs, etc, to ride over and around. Won't be long 'til the summer greenery starts coming out to cover up our winter sins...
As you saw, Rob, all the trails are now fully rideable and in full swing. Blue Mountain was even, dare I say it, a bit *too* dry on Sunday. Stuff that was loamy last weekend was dusty yesterday. Quite a change in the space of one week...