How to prevent the conversion of text URL to link on a single URL?
The only way I think it is possible to selectively disable the "Convert URLs into links" filter is to use CSS and add something like below to the body. Since CSS is applied client-side after Drupal has returned the page, the filter is effectively disabled for this one single link.

&lt;style type="text/css"&gt;
  .mylink &gt; span {
    display:none;
  }
  .mylink:after {
    content: 'http://www.google.com';
  }
&lt;/style&gt;
&lt;p class="mylink"&gt;&lt;span&gt;blah&lt;/span&gt;&lt;/p&gt;


Since you are using the 'Full HTML' format, you should be able to add CSS directly into the body field like shown.