Set this in /etc/sysctl.conf
net.ipv6.conf.all.disable_ipv6 = 1
#apply the settings
sudo sysctl -p
If you just want ipv6 gone from a specific interface change "all" below to say eth0 or whatever interface.
echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
linux, disable, ipv, centos, debian, mint, howtoset, etc, sysctl, conf, disable_ipv, settings, sudo,