Current Ubuntu releases have ipv6 enabled by default and in most cases it isn't needed at least yet. Here's a short description for disabling ipv6 but start by checking if it's even enabled currently.

ifconfig | grep inet6

If that produced no output then ipv6 is already disabled. Otherwise first open (as root) the file /etc/modprobe.d/aliases and locate the following line:

alias net-pf-10 ipv6

and replace it with

alias net-pf-10 off

Then open /etc/modprobe.d/blacklist and add the following at the end of the file:

# disable ipv6
blacklist ipv6

Next you'll have to reboot the system since that seems to be the only way to get already loaded ipv6 modules out of memory.

It should be noted that editing only /etc/modprobe.d/blacklist has the same end result of getting ipv6 disabled but a log entry like

modprobe: WARNING: Not loading blacklisted module ipv6

will get added every time something in the system tries to access the net using ipv6. That can happen often in a busy system so there's no point filling log files with it.