you could use something like
PHP:
$threadModel =XenForo_Model::create('XenForo_Model_Thread');
$threads = $threadModel->getThreadsInForum(2, array(
'last_post_date' => array('>', XenForo_Application::$time - 86400 * 2),
'deleted' => false,
'moderated' => false));
foreach ($threads AS $threadId => $thread) {
if ($thread['first_post_id'] == $thread['last_post_id']){
$dw = XenForo_DataWriter::create('XenForo_DataWriter_Discussion_Thread');
$dw->setExistingData($thread);
$dw->delete();
}
}
this snippet gets all threads from forumId 2, checks if there was a reply, if not, it deletes it
You'll just need an new cronjob with this code