I just gained root access following steps from the http://www.wikihow.com/Become-Root-in-Ubuntu. My prompt get changed from
# sign, indicating I have root access. Now I have come out or log out or disable root access. For that I tried
sudo passwd -dl root. Then it shows the message:
passwd: password expiry information changed.
But I still have root access, and I am still having
# sign instead of
root@hostname:~# still appears instead of