Both women live just far enough away from main branches of the trails that they would seldom see an illegal. All it takes is the right luck with local topography.
Freepers Spiff, AZHSer, and idratherbepainting are not so lucky. They get the constant parade right through their yards. Not their pastures -- their yards.
My only suggestions would be trip wires, fences, electricity, the occasional sneak attack at night with a cattle prod, getting on the roof and setting off LOUD firecrackers, leaving out tainted food (egg salad sandwiches with lots of mayo that have "fermented" in the afternoon sun), etc. What are the chances of retaliation from the Mexican "tourists"?