Short answer: You can turn index entries on or off from the Content tab while editing your book.
Step-by-step: Remove a page from the index
Log in to your account and open your book.
Go to the CONTENT tab (this is the editing screen).
On the left-hand side, select the section or chapter where the page appears.
Scroll down to find the page.
Click the three vertical dots to the right of the page name.
Select Remove from Index from the menu.
The page will no longer appear in your index.
Step-by-step: Restore a page to the index
In the CONTENT tab, locate the same page again.
Click the three vertical dots next to the page name.
Click Remove from Index again so the teal checkmark disappears.
Once the checkmark is removed, the page will be added back to the index.