First you will need to change the root password in your WHM. Then edit the /etc/ssh/sshd_config by setting the PermitRootLogin and PasswordAuthentication values to YES
Then restart your SSHD using this command: /etc/init.d/sshd restart
When you are done with the restart look for the key numbers for users and then delete the allowed keys or edit them.
Then, from your WHM and go to Security Center -> Manage Wheel Group Users and delete irrelevant users