The Joomla installation will create the following in your source code:

meta name="generator" content="Joomla! - Open Source Content Management"

In order to remove the reference to Joomla add following line in the source code of the index.php of your template:

$this->setMetaData('generator','');

Add it direct after the line:

defined('_JEXEC') or die;

Source: http://joom-friends.com/tutorial-remove-joomla-meta-tag-generator-in-joomla-2-5