This is based on some excellent information from http://serverfault.com/questions/286159/how-to-disabled-password-authentication-for-specific-users-in-sshd
Add a Match block to your sshd_config file. Something like this:
Match Group SSH_Key_Only_Users
Or if it's truly one user
Match User Bad_User
See man sshd_config for more details on what you can match and what restrictions you can put in it.