Firstly, check for presence of IPv6 using ifconfig. You should be seeing a few lines containing inet6 addr: ....
To disable, edit a file:
sudo nano /etc/sysctl.conf
Add the following line:
net.ipv6.conf.all.disable_ipv6 = 1
For the change to take effect without rebooting:
sudo sysctl -p
Verify that IPv6 address does not show up in ifconfig.
If you wish to re-enable, change the value in the setting above from 1 to 0 then execute the following two lines:
sudo sysctl -p sudo ifconfig eth0 down && sudo ifconfig eth0 up
If you are doing this over SSH, the current session will of course be cut off. You should be able to reconnect in just a few seconds.