You cannot disable diplomatic interactions for specific people.Pretty sure that's an easy fix if you make it so the Night's Watch can't swear fealty to anyone except the King/Lord of the North, or remove the option from them completely. But the fact this COULD happen isn't that odd considering it was done by the player instead of AI.
As for this bug well it was done by you the player not the ai so honestly not really an issue, just don't be dumb and do things like this and there goes your problem completely!
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