When your website means business.

Custom Admin, Users, WP/CP | Type: PHPForce Private Post

Have used this for user-created content that is not for public.

function force_type_private($post)
{
    if ($post['post_type'] == 'my_post_type')
    $post['post_status'] = 'private';
    return $post;
}
add_filter('wp_insert_post_data', 'force_type_private');