How to remove Meta Generator tag from Joomla 1.5 Print
Many may find the inclusion of this meta tag to be inappropriate for their website. However, determining how to remove it is surprisingly difficult.

The file you want to edit is called head.php and is found here:
/libraries/joomla/document/html/renderer/head.php

you will want to find the following code:
$strHtml .= $tab.'<meta name="generator" content="'.$document->getGenerator().'" />'.$lnEnd;

you can turn it off by simply commenting the line with // at the beginning:
//$strHtml .= $tab.'<meta name="generator" content="'.$document->getGenerator().'" />'.$lnEnd;