[CentOS] restricting access to an NIS netgroup

I am using CentOS 5.7. I have an /etc/security/access.conf file which has
the following:

+ : root : LOCAL
+ : @mynetgroup : ALL
- : ALL : ALL

I thought this is supposed to restrict access to the system to only root
and the accounts in the mynetgroup netgroup; however, anyone NIS account
is still able to login. It appears that the access.conf is being ignored
completely, so I'm thinking there's something I'm missing.

How can I restrict access to a system based on NIS netgroups?

Thanks!

---
Mike VanHorn
Senior Computer Systems Administrator
College of Engineering and Computer Science
Wright State University
265 Russ Engineering Center
937-775-5157
michael.vanhorn at wright.edu
http://www.engineering.wright.edu/~mvanhorn/