DirectAdmin since version 1.39 has function to detect brute force attack (Info) but DirectAdmin didn't block IP that brute force us. It just warn and annoy us by sending an email. DirectAdmin already provide how to automatic block IP when detect brute force for CentOS/Fedora here but they didn't provide for Debian so I will do for you

By theory, SSH (Secure Shell) without password must use private key and public key for pairing between server and client. public key must be put on server and private key must be put on client.

Run below command from client to generate public and private key files

ssh-keygen -t dsa