Once we've eliminated the in-flow at the wall, we can start do transport illegals back to the other side. Most will leave on their own when their jobs are eliminated or their government assistance is eliminated. Not saying it's a sure thing, but let's at least try.
You begin by deporting 10. Then 10 more. Before you know it, word is out and many start self-deporting. Then the employers get the hint and stop employing.
It’s the same way you enforce speed limits on the interstate: pull over a few, most of the rest obey the law.