The cyrus imap server in version 2.2 has a weired behavior: By default an admin user is not allowed to delete mailboxes he created. The good news is, he is allowed to give himself the right to do so. So, if you’d like to create and delete a mailbox you may do it like this:
$ cyradm --user cyrus localhost # enter password for IMAP localhost> cm user. localhost> sam user. cyrus c localhost> dm user.