A document can be unpublished from the register by selecting the Unpublished radio button option in the Details form. 

This will move the document to the Unpublished tab.  Only users with 'Approving Officer' permission levels or higher can access this feature.

Documents in the Unpublished tab can only be seen by users with 'Approving Officer' permission levels or higher. 

Unpublished items can be re-published by selecting the Published radio button option in the Details form.