If you want to disable showing credits for users who has 0 credits, do it this way.
Go to Manage Plugins and edit Display Credits In Postbit:
PHP Code:
if ($vbulletin->options['credits_public'] && !empty($post['credits']))
{
$template_hook['postbit_userinfo_right'] = '<div>' . $vbphrase['credits'] . ': $<span id="cdt_' . $post['postid'] . '_' . $post['userid'] . '">' . vb_number_format($post['credits'], $vbulletin->options['credits_decimals']) . '</span></div>';
}