Militarize the border, heavily. Then start rounding up in-country illegals and putting them back.
Yep, and it doesn't matter if they're Mexican, they go back to south of the militarized border, Haitians, Africans, Arabs, Chinese, what have you. Mexico intentionally let them all cross their nation to get here (helped them, in fact), so they can take care of them when we return them.
