Sunil:
You might want to keep a tire valve tool with you for Quad disabling - it's small, leaves no marks, and does no permanent damage (just 4 flat tires).
All:
When you look at the methods which seem to successfully eliminate presistant trespassers [gunfire, implied threats of death or "Great Bodily Harm" (I'd have loved hearing that back about me - I'm "that Nazi down on Route 37"), heavy law enforcment presence] it's both sad and clear that we're dealing with people who consider themselves above or outside the law. It's sad you should have to do more than have a closed gate and a "No Trespassing" sign to prevent folks from doing this, but those who still do it are the ones who don't care about law or private property - only significant threats to themselves.
I think a large No Trespassing sign with several notches in it placed above an equal number of what were apparently graves, plus one dug-but-unfilled one, would (sadly) be effective.
WRT "local officials":
In addition to our 43 acres, we also make hay on about 12 acres next door. The gentleman who owns this land is VERY worried about liability from trespassers; about 15 years ago he (wrongly) accused my wife of riding her horse on his land (it was some other horsey people from nearby). Since then I have spent a little time trying to keep him calm by putting up "NT" signs for him and blocking access points that lead on to his land from the trespassers' direction (horse riders will apparently not dismount and remove several small trees blocking their intended path in order to trespass).
The only one I have seen trespassing on his property in the last 10 years is an official from the County Engineer's office who was a big HS football star and may still think the world belongs to him (TX residents are probably aware of this phenomenom). He is also the one who "called the cops on the cops" and embarassingly reported my wife's Sheriff's Deputy nephew and his partner, who shoot 200-300 rounds every other week or so on my range, to the local Sheriff's department. (I think my/their frequent and/or heavy gunfire is the reason I do not have trespassers from the subdivision on the other side of us - some of those people are very worried about me. I have offered to have Machine Gun shoots again if they think it would help.) Last Spring he told my wife he was "very upset" that we had completed the line fence on the side of our property along to the land next door we make hay on. (To me, that was a blunt admission of trespassing on us as well as next door.) Nerve!!!
Sorry to rant. As you know, this is a subject that gets your hackles up until you just have to vent.