Hmm, fastjet, I wish it were that simple!! People trespassing are still my responsibility from a liability perspective, sadly!
Thanks for all the comments. I've given this some more thought and read a few PM's about it, and have decided that the best option (and without drawing attention to the fact that it's an airstrip) is to put up some signs asking the users of the right of way to make sure they are completely clear about the proper route of the path ahead of them and not to stray from it.
Not long ago I came to land and there was a family (with dog) wandering up the middle of it. My pass overhead (and then back, and back again....) didn't seem to make them twig, and they gleefully waved up to me. They then carried on ambling along the middle of the strip, casually picking mushrooms and throwing sticks for the dog.
When I finally landed, they stood there with big f***tard grins waving away without any idea whatsoever about the fact they'd made me fly around for 10 minutes while they enjoyed letting their dog crap on my strip.