If you would like to remove all files from your Shareable Content list in one go, you can.
It's important to note that removing these files from this list does NOT remove them from within the showcase itself.
To remove all shared content:
1. Click "Manage Sharing" for the showcase you'd like to edit.
2. You'll see a pop up with your shareable files. On the right hand side of the Shareable on All Slides you will see a small trash can icon.
3. Clicking the trash can will bring up a confirmation pop up. Click Ok if you're sure!
Now you will have no shareable from all slides content available. You can add back in files whenever you like!