Can’t stop automatically adding pages to main nav.

The site is jakeburk.com

I’m trying to set up my portfolio page and what I would like is to have are pages within the Portfolio page that display some of my graphic design work. I go to add new pages, but it automatically adds them to my main navigation links.

I’ve tried simply setting up the parent pages, but they still get added to the main nav.

Does anyone know how I can stop this from happening?

Thank you for your help.