If a site is unresponsive to your DMCA take down request letter, simply find its hosting company and submit it to them. Most Hosting companies are legally bound to respond and remove the infringing material.
Simply type in the sites name on any one of these online resources to find the hosting company. Visit the hosting companies website and look for privacy or legal to obtain contact information to submit your DMCA request. If you do not see any of these, simply submit a support ticket or contact any support email given. Most all Hosting companies will respond fairly quickly upon receiving your request. Be patient, as some take time, but most all will return a response and remove the material if they are hosting the site.