Open the 'showkase/_themes/base/pagetypes/basetheme.tpl' page in a plain text editor and add the following line of code immediately after {block "meta"} at the beginning of the file.
<meta name="robots" content="noindex, nofollow" />
This code will be included in all your Showkase pages and will prevent your site from being indexed (assuming the search engines respect this, which they should).
Alternatively, if your site is running on Apache, you could add the following line of code to your .htaccess file in your Showkase root directory:
Header set X-Robots-Tag "noindex, nofollow"
This would avoid having to modify any Showkase source code.