Sorry for bumping an old thread, but as a note to this if you just replace true with false it won't show the normal thread icon or the poll icon (we have thread's that MUST show the selected thread icon for posts, NWS posts for users at work)
However, if you change it to this it will never show the poll thread icon and only show the normal thread icon (if any were selected).
Code:
// show poll icon
if (0 != 0)
{
$show['threadicon'] = true;
$thread['threadiconpath'] = "$stylevar[imgdir_misc]/poll_posticon.gif";
$thread['threadicontitle'] = $vbphrase['poll'];
}