Your menu is found (the one you have now) in your overall_header template. For phpBB2 it is this {GENERATED_NAV_BAR} If you delete that from your template you'll see the navigation dissapeares.
You can't edit a variable. Thats the menus code, it displays the navigation if you get what I mean.
All you can do is replace it with a a whole new HTML completely and link the HTML to create a new navab. Here, I'll give you and example of creating a new navbar.
PHP is not available and it won't work on Forumotion from the admin panel. If you want to edit the original navbar, you have to access it using javascript. Basic knowledge of HTML and javascript is required for that.