I just gained root access following steps from the http://www.wikihow.com/Become-Root-in-Ubuntu. My prompt get changed from $ to # 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 xyz@hostname:~$.