sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1
sysctl -w net.ipv6.conf.lo.disable_ipv6=1

on the command line to disable it immediatelly (you might need to put all interfaces down and up in order to loose your IPv6 addresses on the interface). The changes will not be permanent and will be lost on reboot. If you need to make them permanent add

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

to your /etc/sysctl.conf.