If your system does not let you su as root, it means the root account has no password. This can be fixed by the following command:
Unless you have used sudo without the past 15 minutes, you will need to authenticate. Use the password of the regular user to authenticate. Then, you will prompted (twice) to enter the password of the root account. Remember to use a strong password!
If your system has su enabled to switch to root, and you want to disable that feature, you should first make sure sudo can switch to root. Otherwise, as soon as su cannot switch to root, you will have no way to switch to root again!
Use the following command to disable the ability to log in as root:
Never execute passwd -d root while you are using su as root. This is because you can end up locking yourself out of the root account for good if sudo is not already set up to let at least one user to use sudo as root.