Hm, unfortunately that is current behavior, if you would like to change it, you can modify on this file and this file cant be override into your current template, mean you have to re-add this modification code in next release version.
JoomlaFolder/components/com_easysocial/controllers/polls.php
LINE 191
if (!$poll->canVote()) {
$message = JText::_('COM_EASYSOCIAL_POLLS_ERROR');
if ($poll->cluster_id) {
$tableCluster = ES::table('cluster');
$tableCluster->load($poll->cluster_id);
$clusterType = $tableCluster->cluster_type;
$message = JText::_('COM_ES_POLL_NON_MEMBER_VOTE_ERROR_MESSAGE_' . strtoupper($clusterType));
}
$this->view->setMessage($message, ES_ERROR);
return $this->view->call(__FUNCTION__);
}