Perhaps it would be better to collect the SSI tax on migrant workers here, immediately transfer that money to Mexico and let them eventually pay it out of their system.
If they paid into it legally and are here legally, then this is the best bet out of all of the responses in the thread. Our tax dollars won't be going to them in the future, they have to goto their government for the money and everybody is happy (except for the Mexicans, who probably won't get their money back from the government, but then, that's their problem and not ours).