On a page that is generated by Joomla, by default you will always see a meta tag like following
<meta name="generator" content="Joomla! 1.5 - Open Source Content Management" />
The easy way is
Go to Extensions >> Template Manager open your default template and click on "Edit HTML"
add following one line of code anywhere between html tags <head> and </head>
<?php $this->setGenerator('Your own meta tags or leave it blank'); ?>
Save and Done
How to remove meta tag "generator" in Joomla
