Again, anyone know how to do this?
The problem being that since the removal of the Mail > Mailboxes page last year, there's no way to directly delete a mailbox AFAICT.
I once before used the trick of creating an address assigned to the mailbox and deleting that address, which deleted the mailbox with it. But this time Create Address doesn't offer the mailbox. Perhaps because the mailbox is a user mailbox rather than numeric mailbox.