I am totally with Rob on this one. The trouble with decent people trying violence as a solution is that they are rarely up for the escalations that indecent or unstable people are all too willing to engage in.
Call the Preserve Headquarters at (845) 255-0919. Put in in yer cell phone where you can find it. Ask for ranger help asap. Put in a 911 call. Make sure the operator understands there is illegal activity with possibly fatal consequences going on. If this is an ongoing problem, the police have to keep hearing about it. If the offenders can be kept there until authorities arrive, so much the better. Get pictures of them if possible. If they can be followed to their vehicle, get make/model/plate numbers.
Beat them up and suddenly you're the criminal, Sparky. Plus it's a tossup whether they leave for good or come back for revenge---unless of course you kill them, in which case your life is over too.
In any case, those of you who have had this experience need to urgently contact the Preserve. It may be time for some signage, and they may need more of a ranger presence up there.