I noticed that using:
imap_setacl ($conn, $mbox, $userid, "")
will act like:
"deleteaclmailbox $mbox $userid" (remove the ACL on mailbox for userid).
(PHP 4 >= 4.0.7, PHP 5)
imap_setacl — Sets the ACL for a giving mailbox
Sets the ACL for a giving mailbox.
Returns TRUE on success or FALSE on failure.
This function is currently only available to users of the c-client2000 or greater library.