While it's very similar to Joomla 1.5.x the removal of the Generator tag in Joomla 2.5.x is a little bit different. Like in Joomla 1.5.x we still have to edit the index.php file of our template in order to hide the Generator meta tag, so just use your favor editor (like Notepad++ or cPanel editor online) to edit your index.php file and add this line :
JFactory::getDocument()->setGenerator('');
For those who don't like to edit their template or they don't know how to do this (you can always ask our support team for help) there's also a simple yet powerful plugin that helps and you can find it in the JED