This is not as far fetched as it sounds; every Macintosh OS X system comes configured in this way: it is impossible to log in as root.
How does one do things as root then? I shall reveal the secret…
First of all, one needs to make sure that the program sudo is available and correctly configured. It must be configured to allow you (or the system owner) to switch to root. Best to test this directly before doing anything to the root account.
Once you have verified that you can switch to root using sudo, then it is time to actually lock the root account. Before doing so, open a root shell using sudo or a direct log in as root. Then execute:
# passwd -l root
There! Now no one can log in as root – don’t you feel much better? Well…. you can become root (by using sudo) but logging in directly as root is impossible.
If passwd does not recognize the -l option, then just put an asterisk (*) into the password field, wherever it is. HP-UX, Linux, and Solaris all recognize the -l option; FreeBSD uses the -l option for a different purpose.
For FreeBSD (and quite probably, OpenBSD and NetBSD as well), use the vipw command to lock out not only the root account, but the toor account as well. The toor account is identical to the root account (including userid) but allows user customization.
When combined with the wheel group, this will lock down your root account quite effectively. Just don’t stop there: remember to use multiple defenses. However, that’s a topic for another day.
Update: This is most useful in situations where a normal user will always have access (workstations come to mind). If your normal users are authenticated via NIS, or Active Directory, or LDAP, don’t do this! If root logins are locked out, and none of the users can log in…….. then what? Uh oh….