How to hide entirely the nav page?

Hi there!

I've hired someone to transform a pdf into an epub. In the file delivered by this person, there's NO nav page (if you wanted to access the navigation, you needed to click on the TOC icon).

But then I had to change a few images and make a few adjustments in the pages myself, using Sigil, and then the nav page started appearing in the book.

I do not want to have that page, it's a children's book anyway.

I've go back to the original file and repeated the process trying to find where that would be triggered, but couldn't find it.

Why is this happening, and how could I hide/remove the nav page?

I appreciate the help! Thnaks!