Here is the steps as outlined by someone on the ssh help mailing list who uses this to limit admin [ root ] access for ssh users:

Easy...  Compile sshd with the match keyword patch
(http://bugzilla.mindrot.org/show_bug.cgi?id=1180),
 and use it to ensure that members of the admin group can only log in using public/private key
authentication.
 
Also make these users a member of the wheel group, and ensure that only  they can su - check out etc/pam.d/su.
 
For example, in /etc/ssh/sshd_config add:
Match Group admins
PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
 
In /etc/pam.d/su ensure the following is uncommented:
 
# Uncomment the following line to require a user to be in the "wheel" group.
auth       required    
/lib/security/$ISA/pam_wheel.so use_uid

I use this method to ensure that admins can only log in using public/private keys and have access to perform admin functions while (hopefully) ensuring that "normal" users cannot mess about.
 
This also has the advantage that if any user uploads their own keys to ~/.ssh that they will not be able to gain admin rights!!!