If you wish to close a discussion to further messages and hide it from the main group screen, you can archive it.
Archive a discussion
Select Discussion Settings > Archive Discussion. It is now closed to further messages.
View Archived Discussions
Once a discussion is archived it can be accessed by clicking on Show All discussions. No new messages can be posted unless it is restored.
Note that Guests added to the group after a discussion has been archived will not be able to see or access the discussion.
Archived discussion messages will still appear within Search results.