If you like to change the robots meta tag in Joomla 1.5, go to line 63-65 in:

libraries/joomla/document/html/html.php file

//set default document metadata
       $this->setMetaData('Content-Type', $this->_mime . '; charset=' . $this->_charset , true );
       $this->setMetaData('robots', 'index, follow' );

To block your site from being indexed and links being followed, change index to noindex and follow to nofollow. Your site's metatag will then appear as
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">

Another option to prevent bots entirely from indexing your site is by adding the text below to robots.txt. robots.txt sits in the root directory of your Joomla site.

User-agent: *
Disallow: /