I think you misunderstand the purpose of host list in the sudoers file. It exists so you can use the same file on multiple hosts while not allowing the same rights on every host.
Instead, you should use ssh keys to allow passwordless login as root. You can create two keys for the user in question. Make one of the keys encrypted with a passphrase, and keep the other one clear text. On your server, put the public keys into root's .ssh/authorized keys file. Then prepend
from="a.a.a.a" to the passwordless public key's line to prevent anyone from using it from another host.
This requires you to allow root login over ssh, but you should only allow root login with ssh keys. Make sure your sshd_config has the following setting: