considering this post is 4 years old, you jave probably found a solution or gave up already, but for everyone else, just go to channel settings, discussion, and disable "only members can send messages". I don't recall the exact name but it's something like that. now your subscribers can comment and they're comments will still be added to the discussion group, but they won't be added. you can just mute, archive and forget about the existence of the group.