Unless otherwise indicated, the code snippets you see below should be placed into your theme’s functions.php file.
Here is the code to unregister the primary navigation menu:
<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.
//* Unregister primary navigation menu
add_theme_support( 'genesis-menus', array( 'secondary' => __( 'Secondary Navigation Menu', 'genesis' ) ) );