I recommend using SSH key based authentication to all your Unix servers. There's a good rationale for this written here.
To ensure users use SSH keys to log into your server, turn off
PasswordAuthentication
like so:
# echo PasswordAuthentication no > /etc/ssh/sshd_config.d/disable-password-login.conf
# systemctl reload sshd
Now, if the user doesn't have a private key that matches the entries
in ~/.ssh/authorized_keys
on the server when logging in, he'll get
no password prompt anymore:
$ ssh foo@ssh.example.com
foo@ssh.example.com: Permission denied (publickey).