Hi!
Are you comfortable with editing the code of your themes? Unless your theme has the option to disable the navigation menu, you’ll have to edit the code (which can be done with WP’s built-in editor, under the Themes menu).
In your theme’s index.php file, there’s a certain snip of code that pulls in the nav menu. It looks something like this:
<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?>
Read more about it here.
All you need to do is “comment it out”, by turning this line into a comment. So your code would look like this:
<!-- <?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?> -->
Notice the extra bits I added before and after the line? These make it a comment, so WordPress will ignore it when building your website.
Good luck!