How To Disable ipv6 on Ubuntu 8.10

sudo perl -pi -e 's/ipv6/off/g' /etc/modprobe.d/aliases

And simple restart the network

sudo /etc/init.d/networking stop
sudo /etc/init.d/networking start

Disable IPV6 on ubuntu 9.04

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo /etc/init.d/networking restart