Login to your server as a user with sudo capabilities.
Once logged it, you will want to edit the /etc/ssh/sshd_config file with your favorite editor to prevent root access (I chose nano):
sudo nano /etc/ssh/sshd_config
Look for the line that contains PermitRootLogin yes and change it to the following:
PermitRootLogin no
Save your changes and then restart SSH:
/etc/init.d/ssh restart