We've added a new tab type called "Archive". This new tab type was requested by several users that wanted to close a tab to discussion, but not delete the tab. The basic function of this new tab type is to make a tab private and make it read-only.
Existing users are still members of a tab that has been archived and can use the Archived tab for reference. The organizer of the tab can change the tab back to an active tab to renew the discussion.
Use the Manage menu > Security to make changes to your tab type.