In one of the /conf/ files ( _advanced.php I *think* ) there's a setting called $require_name_email .... or summat like that, just set it to false ;)

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*edit*
Damn I'm so cool ... almost, right file, right variable ... and I'll claim false will work anyway, but it should be 0

/**
 * Comments: Set this to 1 to require e-mail and name, or 0 to allow comments
 * without e-mail/name.
 * @global boolean $require_name_email
 */
$require_name_email = 1;