Well, the code generating the google ads on that page is about halfway down the page and start with the javascript code that includes the google_ad_client id code.
<script type="text/javascript"><!--
google_ad_client = "pub-5098820216071248";
PHP-Nuke is a content management system (CMS) much like Joomla (but nowhere near as good), so it is probably using a module to include the Google Ads on the page. You need to log into the administrators backend of the php-nuke install and look through the installed modules and either disable or uninstall the one generating the google ads. BTW, who's google account is the advert revenue going to?

Slightly unrelated, but how old is the version of php-nuke being used on that site? I noticed it has a copyright of 2002! but the latest release was last year and version 8.0 which has a lot of security improvements. I would highly recommend considering upgrading to a newer version of php-nuke due to the number of bad security problems with that software.