Well, you could try this from an admin controller:
PHP:
// deleted forums:$nodes = array(1, 2, 3);
$threadModel = $this->getModelFromCache('XenForo_Model_Thread');$imodThreadModel = $this->getModelFromCache('XenForo_Model_InlineMod_Thread');
$threads = $threadModel->getThreads(array('node_id' => $nodes));
$threadIds = array_keys($threads);
$imodThreadModel->deleteThreads($threadIds, array('deleteType' => 'hard'));
I think that will do the job...