That's not as hard as it seems. The majority of the money is wired through holding companies like Western Union. It's easy to say no funds can be wired to Mexico.
Is it feasible to put them immediately on planes, trains, buses, and ships back to their countries of origin? Let those countries sort it out at their own borders. It seems to me that it would cost a hell of a lot less and have more of an immediate impact.
It would be money well spent to imprison people crossing the border illegally. Others who might be inclined to cross the border would see a severe penalty when they get caught.
Is he proposing that Congress pass a constitutional amendment that the states ratify? If so, great, let's clarify that. If not, what is the proposed method?
It's going to be hard to pass a constitutional amendment to end birthright citizenship. I doubt the SCOTUS would back a law ending it.
So they buy VISA gift cards and drop them in the mail or use PayPal. I'm not saying it's impossible, I just don't think it's as easy as that. How do you ensure that American citizens can still wire money? Driver's License isn't the answer.