Exclude custom post_type in sitemap generation?

If you are using functions.php script to register custom post type, you should declare false to 'has_archive' => true,.

function custom_post_type() {

  $labels = array( ... );
  $args = array(

    // you have to set it to False.
    'has_archive' => false,

  );
  register_post_type( 'post_type', $args );

}
add_action( 'init', 'custom_post_type', 0 );