To disable ssh root access you need to edit the ssh configuration file, on Debian it is /etc/ssh/sshd_config, to edit it using nano text editor run:
nano /etc/ssh/sshd_config
On nano you can press CTRL+W (where) and type PermitRoot to find the following line:
#PermitRootLogin prohibit-password
To disable the root access through ssh just uncomment that line and replace prohibit-password for no like in the following image.
After disabling the root access press CTRL+X and Y to save and exit.
The prohibit-password option prevents password login allowing only login through fall-back actions such as public keys, preventing brute force attacks.