Turn off SSH access for a specific user in Ubuntu by modifying the SSH daemon configuration file

Edit the SSH Configuration File

Open the main configuration file using a text editor like nano with administrative privileges

sudo nano /etc/ssh/sshd_config

Add the Deny Directive

Scroll to the bottom of the file and add the DenyUsers directive followed by the username you want to block

DenyUsers username_to_block

Restart the SSH Service

Save the file (press Ctrl+O then Enter in nano) and exit (Ctrl+X). Test your configuration for errors, then restart the service to apply the changes

sudo sshd -t
sudo systemctl restart ssh

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Alternative: The Whitelist Approach

If you prefer to block everyone by default and only allow a few specific users, use AllowUsers instead

AllowUsers allowed_user1 allowed_user2