Any provision of any rule or regulation under the Act which prohibits any act, practice, or course of business by any person if the mails or any means or instrumentality of interstate commerce are used in connection therewith, shall also prohibit any such act, practice, or course of business by any broker or dealer registered pursuant to section 15(b) of the Act, or any person acting on behalf of such a broker or dealer, irrespective of any use of the mails or any means or instrumentality of interstate commerce.