For one, get a picture of their vehicle, with license plate, if possible, then call the game warden and demand they be ticketed for violations. (Or, start out nicely, by *suggesting* they get ticketed.)
If you can afford it, look into some kind of perimeter system which will shock the living daylights out of them if they come in contact with it.
Rig up alarms/flashing lights/etc to any gates, entries onto the property, etc. which will instantly alert you and, again, scare the living daylights out of them.
Set up loudspeakers and get a recording of random gunshots to play in a loop or an authoratative voice, like the robot on Space Family Robinson**bg**, which says "Warning, Warning, you have violated the NO TRESPASSING ZONE, LEAVE IMMEDIATELY!" But, I agree the violin would be better and you don't want to scare all of the deer off of your safe property, either.:-)
Good luck! Maybe Alba will stop i this thread. I think she's had to deal with this kind of problem,too.