At last I found the solution:
I have added these codes in the qa-theme.php file after the first "{".
function head_metas()
{
if ($_SERVER['REQUEST_URI'] == '/login') $this->output('<meta name="robots" content="noindex, follow">');
if ($_SERVER['REQUEST_URI'] == '/register') $this->output('<meta name="robots" content="noindex, follow">');
if ($_SERVER['REQUEST_URI'] == '/admin') $this->output('<meta name="robots" content="noindex, follow">');
if ($_SERVER['REQUEST_URI'] == '/faq') $this->output('<meta name="robots" content="noindex, follow">');
qa_html_theme_base::head_metas();
}
This will add meta info to the requested pages.