31 U.S. Code § 5363 - Prohibition on acceptance of any financial instrument for unlawful Internet gambling
No person engaged in the business of betting or wagering may knowingly accept, in connection with the participation of another person in unlawful Internet gambling—
(1) credit, or the proceeds of credit, extended to or on behalf of such other person (including credit extended through the use of a credit card);
(2) an electronic fund transfer, or funds transmitted by or through a money transmitting business, or the proceeds of an electronic fund transfer or money transmitting service, from or on behalf of such other person;
(3) any check, draft, or similar instrument which is drawn by or on behalf of such other person and is drawn on or payable at or through any financial institution; or
(4) the proceeds of any other form of financial transaction, as the Secretary and the Board of Governors of the Federal Reserve System may jointly prescribe by regulation, which involves a financial institution as a payor or financial intermediary on behalf of or for the benefit of such other person.