Hiding pages from Nav Menu on Three-Column-Blue theme

I am using ‘three-column-blue’ theme on my website
https://www.rockpaperscissorsconsulting.com

Each time I add a page to this website, it automatically adds it to the navigation menu. I can’t find a way to hide pages, which is proving a problem when wanting to store a video on our site that is only accessible once you sign-up to our mailing list.

I’ve worked out how to ‘password protect’ things, but that is not feasible for this option.

Can you suggest how I can hide pages on my website?

Thank you in advance.