Once a flipbook has been published as public, it cannot be changed to unlisted, as the flipbook is already public on the internet, and has been indexed by search engines.
To solve this, you can instead:
1. Make a copy of the old flipbook.
2. When you choose to publish, make sure that you select the unlisted option.
3. Delete the old flipbook.
Important: The statistics from the old public will not carry over to the new flipbook.
Once deleted, people who have already seen the public versions will no longer be able to access that publication.