Every promise made after August 21, 1933, between any employee or prospective employee and his employer, prospective employer or any other person is contrary to public policy if either party thereto promises any of the following:
Such promise shall not afford any basis for the granting of legal or equitable relief by any court against a party to such promise, or against any other persons who advise, urge, or induce, without fraud or violence or threat thereof, either party thereto to act in disregard of such promise.
Ca. Lab. Code § 921