Having walked down the odd green lane the majority of the damage to tracks appears to be from quads and trail motorcycles.
While conventional barriers will stop the larger vehicles i feel that they will be no barrier whatsoever to these types of vehicle which by there very nature can squeeze through small gaps (which then get wider) or with two individuals almost any trials bike can be lifted or with some hopped over lower obstacles. Indeed this hopping forms another part of the sport.
Proper trials clubtend to use prepared areas (been a side car rider in the past) with concessions from local farmers etc.
"If you can't be a good example, then you'll just have to serve as a horrible warning."