IPv6 isn’t as popular as it should be, and within a home network concept, it’s not really useful.  Unless you want to learn how it works.

With Kubuntu, the /etc/sysctl.conf file holds the key. Adding the following line at the end of the file and then rebooting, will disable IPv6 from your network connection setup:

net.ipv6.conf.all.disable_ipv6 = 1

I have tested this from  version 10.04 until the current Kubuntu version.

My philosophy about this is if I do not use something, why should it be running.  I like to run a clean environment and knowing what lies behind the curtains.