$db->setQuery("DELETE u FROM #__mycomponent_users AS u
                LEFT JOIN #__users AS ju ON u.userid=ju.id
                WHERE ju.username IS NULL");

$db->getQuery(true)
            ->delete($db->quoteName('#__mycomponent_users', 'u'))
            ->leftJoin($db->quoteName('#__users', 'ju') . ' ON u.userid=ju.id')
            ->where('ju.username IS NULL');
$db->setQuery($query);