If some random guild clerk can ban with no authorization from the guildmaster, it sounds like this system is capable for abuse that the author never even considered being a possibility, or this is a thing that is usually kept a secret for problem members of the guild. But even if you hated someone who was listed as A-rank adventurer, the fact that some civil servant can lock any adventurer out of a job sounds like it would need at least 3 tiers of confirmation to ensure major walking military might of the nation don't just up and leave at any time.