So I accidentally banned a guest from the clan chat and as the ban list is full it automatically unbanned him after a set amount of time. Since then, we get a notification that he is joining but he is not listed in the Clan Chat and he is unable to see any chat.
This has happened to the individual previously but it resolved its self after some months.
Stay calm, and use defensives.
It sounds like he is on the ban list, and you get the message each time he attempts to join - but doesnt actually get into the chat. This could be happening with him auto joining your clan chat each time he logs in, and so will only stop once he either removes himself from guesting or guests in another clan.