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
   

   How to restart SSH service