Disable an account
Modify the appropriate user you wish to disable from /bin/bash to /bin/disabled
***This option will NOT prevent login over SSH***
Modify account password or lockout account
passwd -l <<username>>
The -u argument instead of -l will set the account as unlocked.
Curious for more details on who has attempted logon?
Keep in mind, aside from locking a user out, it is equally important to make sure you properly lock down your firewall to expose ports like 22 only to specific IPs. Don’t leave yourself exposed to the world.