The Great Wall of China was a dumb idea? The Berlin Wall and the wall around Jerusalem and the Vatican are dumb ideas too? You are so naive. Submarines under the Rio Grand? LOL
The coast guard has seized several submarines full of cocaine. If it was easy to control your borders why do we have drugs on our streets? Here is how capitalism works. If something is needed, someone will find a way to make a profit providing it. The coyotes, cartels etc. have a lot of capital to play with and it will not be simple, easy or cost effective to stop them. If you want people to stop coming into your country to get free stuff, maybe you should stop giving them free stuff.