I think getting a bull might help, put up lots of signs and move it around the fields a lot, possibly somone getting gored will keep others away, although the risk of the bull getting shot by one of these idiots is probably high. Also try electric fencing around your property. It stops most people!
Clamp their trucks if they are parked on your land (you can buy wheel clamps on the internet I believe) and leave them the number of the local police station, let them explain why they are trespassing on privite property! Or set up traps which will rip their tyres to shreds. They might get the message.

You could always get a pack of wolves to roam your land (Impractical I know, but it is a nice thought).

Not sure how possible any of these suggestions are, but they would probably be great and even if you couldn't use any of them imiganing the look on peoples faces would be great, plus they are pretty untracable, no-one could really blame you personally!