Easy:
50% tax on Western Union remittances and Mexican bank transfers.
PayPal transfers also .
I'd go higher than 50%.
The outrage of Mexico's elites towards Trump's policies has nothing to do with their BS talk of "human rights" or "racism" and everything to do with the fact that remittances account for a significant chunk of Mexico's GDP (it's even higher for many poorer Central American countries), combined with the fact that Mexico needs an outlet for a surplus population that its economy and infrastructure can't support.