Warning: If you issue the command

sudo passwd -l root

you will not be able run Cron jobs as user root or add new users.
You will see "Authentication failure" messages in your syslog because the root account is marked as expired.

To fix this you have to edit your shadow file manually:

sudo vi /etc/shadow

Change the line similar to:

root:!:13919:0:99999:7::1:

to

root:!:13919:0:99999:7:::