Elaborating on the answers above.
This fellow used SELinux to secure a machine with public root access as a proof of concept.
SELinux - http://www.linuxjournal.com/article/6836
However, SELinux is an adventure to learn.
You could use the
chattr +i above and just not tell them about it...depending on how clever your users are.
chattr -i would allow any root user to edit the file. In addition
+a allows only appending.
The "normal" way to do this would be to create normal users and specifically give them the privleges they should have via something like the free tool