To turn off an exhibition click on the event you don't want to show, and click the pause button on the menu across the top of the exhibitions.  You will see a pause icon now appears on the exhibition and it will be faded out.

If you highlight the exhibition again you will see the pause icon on the top menu is now a play button.  To reinstate it to your website you need to click on the play button and the exhibition will now be live on your site.