Remove custom submenu pages

functions.php

// You need to find out the parent menu slug, used when added the submenu:
// add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function );

// Replace those next
function remove_submenu() {
    remove_submenu_page( '$parent_slug', '$menu_slug' );
}
add_action( 'admin_menu', 'remove_submenu', 999 );