Users can join in to not published groups

Even if new group is not published (checkbox "Published" in "Publishing options" not checked) or group don't appear in the groups directory, users can join in to this group by using link like this: http://example.com/og/subscribe/13?destination=og. I want to premoderate all new groups, but users, who know about this bug, can create and post in to new group without premoderation.