SSH in most system by default allow login using password. While this is the common setting, there are time that the users need to be enforced to used stronger means of authentication, such as passphrase protected public key. The option to allow password login is  PasswordAuthentication in the /etc/sshd_config configuration file. If it is not set, SSH daemon will allow password based login.

To disable it, add the following line (or change the option to no if the option already exist) in the configuration file;

PasswordAuthentication no

To have the changes take effect, the SSH daemon need to reload the edited configuration file with the following command;

# /etc/init.d/ssh reload

In some Linux distribution, the command is as the following;

# /etc/init.d/sshd reload