Disable menu in header

I have used the avada theme for a website https://www.fotofonkeling.nl.

It has a custom menu, but when i switch to a responsive version of the site, the built in dropdown menu is messed up. This happens because i didn’t assign a menu to the main navigation, as that will make it show up in the header. I obviously dont want this.

How do i get this right? I can assign the menu to the main navigation, but i have to stop the header from getting the menu somewhere.

Any help is greatly appreciated.