You need to add custom css to remove the div for the menu, but not every theme will identify the menu in the same way, so there’s no way to be sure the same custom css will work for every theme.
However, I’d get a plugin like My Custom CSS, find the css code that controls the menu in your theme (let’s say it’s called #menu1) and give it a “display: none;” — like this:
#menu1 {display:none;}
If you change the theme, the menu might now be called something like #navbar, so you have to find the proper part of the css again, and then change your custom css to:
#navbar {display:none;}
But I’m not an expert, so perhaps you will get better advice than this!