No attorney, certified public accountant or enrolled practitioner shall, in practice before the Bureau, knowingly and directly or indirectly:

(a) Employ or accept assistance from any person who is under disbarment or suspension from practice before any agency of the Treasury Department;

(b) Accept employment as associate, correspondent, or subagent from, or share fees with, any such person;

(c) Accept assistance in a specific matter from any person who participated personally and substantially in the matter as an employee of the Treasury Department.