Isn't that sort of like racketeering and just what the RICO law was made for? Sure wish Elliot Ness Spitzer was as gungho about busting illegals as he is cigarette smokers.
Beyond that if you manage to fire them and they come back with papers a few months later, "Look I am legal now!" you will likely have to hire them back. Praying that this time the papers are legitimate. The union does not consider violation of the immigration law to be a good enough reason to permanently fire someone.