Try using visudo to edit sudoers and remove that line.
I bet sudo would then block root. So, `sudo ls -l` by root would fail; whereas, obviously, `ls -l` directly by root would work just fine. You can repair the "damage" by doing `vi /etc/sudoers`. vi just won't syntax check for you when you are done, and you could break things if you mess up.
On my Mac OS X system, that same line appears with %admin. So, an admin user can do anything with sudo.
Full disclosure: I haven't tried it, and don't want to mess with my servers. But it seems perfectly logical.