Go to the WP-Admin section of your site, and click on Appearance > Editor.
On the right side, click on Header.php.
Find the part that says “<?php wp_page_menu(‘show_home=1&depth=1’); ?>” OR “<?php wp_list_pages(); ?>” or something to that degree… and REMOVE that.
You might want to hold on to a copy of that just in case you screw it up and you can put it back together. Also, I’d recommend backing up your data to your computer, to be safe.
Good Luck!