I'm not sure you should disable login for root, but it is possible to open this for other users. Just log in as root, and use chsh to change the shell of the user to /bin/bash (chsh -s /bin/bash user). The current shell is /bin/false.
If you were to change the root-shell to /bin/false, then root wouldn't be able log in, but I'm not sure if there are any big disadvantages of doing this.
- What would happen if the system crashes and you boot in single user mode ?