Source: https://www.leowkahman.com/2016/03/19/disable-ipv6-raspberry-raspbian/
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