To disable the root account, use the following passwd syntax:
sudo passwd -l root
-l, --lock Lock the password of the named account. This option disables a password by changing it to a value which matches no possible encrypted value (it adds a '!' at the beginning of the password). Note that this does not disable the account. The user may still be able to login using another authentication token (e.g. an SSH key). To disable the account, administrators should use usermod --expiredate 1 (this set the accounts expire date to Jan 2, 1970). Users with a locked password are not allowed to change their password.