The key probably is freebies. End the free health care, charge them for services, and many will come back home here to Mexico. But if you ARE going to do freebies, then build a wall. It won't stop them, but it WILL make someone feel good.
Indeed, ending the freebies is a big key. But it needs to be combined with some other keys -- huge fines for employers who hire illegals, and and end to the anchor baby law.