1. buy bigger/faster bike so you can catch them

2. NO Tresspassing signs

3. Document the damage and send them a bill for the damages and when they ignore the bill
   file in small claims court.

4. Buy an old dirt bike (cheap)  and either drive over it repeatly with a tractor or,, take
   something like a shotgun  and blast the krap out of it.  THen... place the beatup-shotup
   Dirt bike next to the road with a sign,, "this is what happend to the Last dirtbike who
   some
   SOB decided to ride on my land and destroy my property".

Should solve the problem with any 3 of the suggestions.

(depends on your local laws of course..)

.