One guy on another forum came up with a good answer.

Put up some signs on your property - no tresspassing - rifle range.

Then put up some back stops and some targets on your property - around where the 4 wheelers like to ride.

Then when you see them coming, give them one verbal warning.

The next time - let loose with a 50 BMG at the targets and see how long the kids with the 4 wheelers and the dirt bikes stay around.

Instant remedy forignorant people - scare the chit out of them.

They will not come back!