A 20 ft razor wire topped double fence with machine gun nests every 100 yards and packs of vicious man eating guard dogs patrolling the no man's land in between the fences round the clock ought to do the trick. Any one who gets through either way gets gunned down or eaten on the spot. Round up the 15,000,000 already here. Rip them out of their homes and shops in the dead of night (so the neighbors won't panic) and load them onto transports headed south. But 15,000,000 people sounds like an awfully large number of people to transport to deportation stations. This could present a bit of a problem. Buses and planes are out of the question. Too small and much to slow. Hmmmmm... how about freight trains? That's it. Load 'em up in box cars and head'em south. That ought to do it.