You can easily Archive a Room once you have finished that project room or a section of work that you no longer want people to participate in. Archiving will allow previous participants access but they can no longer add content. 

Once it is archived participants can still view the content and copy the content from the archived room! 

Want to archive a room - you can do so from the home page or from within the room.
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On the home page or rooms directory click the three-dot menu (ellipsis) and select "Archive Room" 

Inside the room, go to Room Settings and click "Archive, 

While Archived participants can view and copy content they can no longer add content to the room.  To allow for further participation (want to keep working or add new content) you will need to "Reactivate" the room.

From the home page or rooms directory click the three-dot menu (ellipsis) and select "Reactivate Room" from the menu.

Or view the room and select "Reactivate room" from the overview page within the room.