function demo_settings_page()
{
    add_settings_section("section", "Настройки страниц:", null, "demo");
    add_settings_field("demo-checkbox", "Закрытить внутр. страницы", "pages_404", "demo", "section");
    register_setting("section", "demo-checkbox");
}

function pages_404()
{
   ?>
        <!-- Here we are comparing stored value with 1. Stored value is 1 if user checks the checkbox otherwise empty string. -->
        <input type="checkbox" name="demo-checkbox" value="1" <?php checked(1, get_option('demo-checkbox'), true); ?> />
   <?php
}

add_action("admin_init", "demo_settings_page");

function custom_settings()
{
  ?>
      <div class="wrap">
         <h1>Дополнительные настройки</h1>

         <form method="post" action="options.php">
            <?php
               settings_fields("section");

               do_settings_sections("demo");

               submit_button();
            ?>
         </form>
      </div>
   <?php
}

function menu_item()
{
  add_submenu_page("options-general.php", "Дополнительно", "Дополнительно", "manage_options", "custom-settigs", "custom_settings");
}

add_action("admin_menu", "menu_item");


$option_404 = get_option('demo-checkbox');

if(isset($option_404) && $option_404 == 1)
{
    add_action('template_redirect', 'test_redirect');
    function test_redirect()
    {
        global $page, $wp_query;

        if(!is_front_page())
        {
            $wp_query->set_404();
            wp_redirect( '/', 404 );
        }
    }
}