in Order to block crawlers of the search engines like google from reading and indexing sites of your wordpress installation you may use the slug.
If you want to block the site www.yourdomain.com/impressum/ for example you put the following code into your header:
<?php if (is_page('impressum')):?><meta name="robots" content="noindex " /><?php endif;?>
if you use your own robots.txt, your may add the following to it
User-agent: * Disallow: /impressum/