the RFL Operational Rules are suitably vague in this area:
The Board on behalf of the RFL shall be entitled to refuse registration of a Player or refuse to accept a Contract, Contract renewal, variation or extension in any of the following circumstances:
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(b) the Board reasonably believes that to accept the registration or contract may bring the Game into disrepute or may adversely affect the integrity of any competition, or
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(f) the Board reasonably believes that the Player is under a relevant suspension or ban imposed by the RFL or another governing body of any sport in any part of the world.
So basically the RFL can choose to accept the ban or not.