Assimilation is an impossibility when you're dealing with a people who view a large portion of the U.S. as Mexican land. They have no intention of ever assimilating, and they never will (despite our best efforts).
Again, providing incentive for them to go back home (and to never come here in the first place) is the only answer.
It might be your "only answer". But it isn't going to happen.
So now what?