To archive a discussion, find the discussion card in the portal (see: Finding a Discussion). Click on the three dots that appear at the top-right-hand of the discussion card when you hover over it, and select the ‘archive’ icon. A warning window will pop-up, click ‘OK’ to archive the discussion.
To unarchive a discussion, follow the same steps, but select the ‘unarchive’ icon.
When you archive a discussion, it is frozen - i.e. everyone who has been invited to the discussion will still be able to access it, but no more changes can be made. To entirely remove an archived discussion, see Deleting a Discussion.
Only the Owner and any Admins can archive a discussion. Only private discussions can be archived (see: Public vs Private Discussions).