Recommended course:

Complete Secure Shell Course

SSH in most system by default allow public key login. You can disable it with these simple steps if you somehow need to;

  1. Open SSHd configuration file with your default editor.
    $ sudo vim /etc/ssh/sshd_config
  2. Search and set PubkeyAuthentication option to no. Add the line if it doesn't already exist.
    PubkeyAuthentication no
  3. Reload or restart SSHd