First you remove this line from /etc/modprobe.conf if this line exists.

alias net-pf-10 ipv6

Add the following line to /etc/modprobe.conf

alias net-pf-10 off

Reboot the system. IPv6 is now disabled.

To re-enable IPv6, remove the alias net-pf-10 off line from /etc/modprobe.conf and reboot the machine.

I’ve also made a post about how to disable IPv6 on RHEL5.