How to disabel main menu??

I whant to disabel the main menu that comes with the theme how do i do that?
i just whant the primary menu enabel, because the main menu i cant add a sub drop box to create a categories i have woocommerce and i whant to put product categories in main menu exp: home, contact us , catagories and under categories it will show a drop box that says the categorie like Sport & health thats what i want but i only can do that in the primary navagation.

unique-gadgets.com
thanks in advance