Hide sub pages from top nav

Use the depth=1 argument with the template tag, wp_list_pages().

The template tag, wp_list_pages(), is typically used by theme designers to display Pages in a nav bar. With the help of the Template Hierarchy article, determine what Template is displaying your Pages (probably header.php), then look for wp_list_pages and add the depth=1 argument.

Please, review the wp_list_pages() article.