Originally Posted by earthnet
In your /etc/ssh/sshd_config file make sure that PublicKeyAuthentication is "yes" and PasswordAuthentication and UsePAM are set to "no". There might be another directive or two that I'm not remembering off the top of my head. Try it and if it let's you in without a key, then there is another directive to disable.
There is the option in /etc/ssh/sshd_config
Contrary to the first impression you get from this option, it will limit root logins to be done by public keys only. In addition it could be limited to be allowed only from certain machines with an AllowUsers
option in addition therein.