I am configuring a CentOS 5.2 box to act as a gateway. After doing the initial configuration I want the ADSL connection that I configured not to try and start when doing a /etc/rc.d/init.d/network restart or service network restart.

This may seem like a no brainer but I'm posting it because it wasn't obvious to me.

The answer:

Edit the ifcfg-ppp0 file and change the ONBOOT parameter to no. Which means the network script will skip it.

vi /etc/sysconfig/network-scripts/ifcfg-ppp0
ONBOOT=no

This was done on:
lsb_release -d
Description: CentOS release 5.2 (Final)