Plant indigenous bushes to the desert that have thorns of at least one-inch in length on either side of the border (about a quarter-mile each way) and use dust cropping planes to spray each side with a non-lethal insecticide daily (that may turn lethal if they try to burn the bushes). Maybe like a long-lasting skin irritant. Then have the Mexican government issue warnings/post signs. "Do it the cost-effective, non-lethal and enviromentally reckless way". Ju know?