Get a solicitors letter about trespass, and enlarge and laminate it, then display it at the point you'd turn off the footpath and bridleway onto the strip?
I did have trouble (before the barbaric practice got deservedly banned) with hare coursers on our strip runway a few times. When I confronted them, they got very argumentative. When however I pointed out the likely consequences of close contact between one or two of their precious greyhounds and one of our propellers (and lied a bit about how little real control we have over, and visibility from, a landing aeroplane once established on finals), they simply went away. You could try something like that.
G