functions.php

<?php

/**
 * Limit Primary Menu to Top Level Items
 * 
 * @author Bill Erickson
 * @link http://www.billerickson.net/customizing-menu-arguments/
 * 
 * @param array @args
 * @return array
 *
 */
function be_primary_menu_args( $args ) {
  if( 'primary' == $args['theme_location'] ) {
    $args['depth'] = 1;
  }
  
  return $args;
}
add_filter( 'wp_nav_menu_args', 'be_primary_menu_args' );