HOWTO:
There are many different, alternative, or strange ways people tell you that it can disable IPv6.
But the best way is using /etc/sysctl.conf to disable it in your kernel.

   Using those commands to append kernel settings/parameters into /etc/sysctl.conf
echo "#disable ipv6" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.all.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.default.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.lo.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf

Enable those settings Righ Now!

sysctl -p

Done!
All those setting will will be kept and will be applied/enabled automatically at future boot time.


Tested on: Ubuntu 8.04 LTS, Ubuntu 10.04 LTS