Allowing only root user to login through ssh

This can be done in two ways, not sure whether it's possible in another ways. Here are the two possible ways.

1. Configuring /etc/ssh/sshd_config file

AllowUsers root - which will allow only root user to login through ssh

2. Using PAM.

Add "Account pam_access.so" to /etc/pam.d/sshd 

Adding the above the line make login process to check /etc/security/access.conf for access

-:ALL EXCEPT root:ALL