You have see the sysctl command in the tutorial about restarting network in Ubuntu. We are going to use it here as well. To disable IPv6 you only have to input 3 commands:
sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=1
You can check if it worked using:
ip a
You should see no IPv6 entry: