get'em going the wrong way across the border..
..and then seal that sucker
http://farm4.staticflickr.com/3585/3460882920_6f052e2f43_z.jpg
http://2.bp.blogspot.com/_r9eiPBELMBM/R3LaNHrthrI/AAAAAAAAAlY/hWYprbdfDI8/s320/Welcome%2Bto%2Bmexico%2Bsign%2Barrow.jpg