In NLRB v. Textile Workers, supra, and Machinists v. NLRB, 412 U.S. 84 (1973) (per curiam), the Court found as a corollary that unions may not fine former members who have resigned lawfully.
Observing that the NLRB does not require that a union have actual knowledge of an employee's resignation from the union before the resignation is deemed effective and noting a case where the NLRB held that "the failure of the union to pick up the letter until several days later should not affect the employee's right to resign"