The National Labor Relations Board has held that any union restriction on a member's right to resign or otherwise refrain from engaging in Section 7 activities is invalid.
International Association v. Machinists, Local 1414, 270 N.L.R.B. 1330 (1984)
Labor Law - National Labor Relations Act - Section 8(b)(1)(A) - Union Restriction on Resignation and Post-Resignation Conduct,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol24/iss3/11