Disable ssh password login

Add a sshd config file with the following content 

 sudo nano /etc/ssh/sshd_config.d/99-ts.conf 

 ChallengeResponseAuthentication no

PasswordAuthentication no

UsePAM no 

 Reload the SSH daemon 

 sudo systemctl reload sshd 

 WARNING! Now you are no longer able to create new session to that server! Remember to add your ssh certificate before closing the session!