this is a touchy subject but i also believe there is a solution to your needs - not really sure why you need to do this though unless you're concerned that your site will be found based on the tag
the code that generates this is in frontend.php. the code is
$mainframe->addMetaTag( 'Generator', $_VERSION->PRODUCT . " - " . $_VERSION->COPYRIGHT);
i would recommend commenting it out and not deleting it as there is a requirement in the joomla license that the source code has to be copyright and although this generates code and isn't actually a copyright notice its a thin line
if you want real clarification on what you can and can not do though i would suggest asking over at
http://forum.joomla.org
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