How about taxing the western union and other ways that they send money back to relatives?
Financial Access for Immigrants: The Case of Remittances
More important for remittances, in February 2004 the Reserve Banks expanded their international ACH services to Mexico, in cooperation with the Central Bank of Mexico. The service potentially connects any bank account holder in the United States with any bank account holder in Mexico, uses an exchange rate guaranteed to be within 1 percent of the Central Bank of Mexico's wholesale rate, and costs the banks less than $1 per transaction. Providing service to Mexico is also an important step for the U.S.-Mexican Partnership for Prosperity, an agreement designed to improve financial linkages between the two countries. These Federal Reserve initiatives will support U.S. banks' ability to serve immigrants by allowing remittances to be sent to foreign banks at low cost. Ongoing improvements in the infrastructure for sending remittances, collaborations among foreign governments, and increased competition among service providers should ensure that cost savings are passed on to consumers.
Remarks by Governor Ben S. Bernanke
At the Financial Access for Immigrants: Learning from Diverse Perspectives conference, Federal Reserve Bank of Chicago, Chicago, Illinois
April 16, 2004
http://www.federalreserve.gov/boarddocs/speeches/2004/200404162/