I'm not bought in to using IPv6 yet and frankly, I haven't messed with all of its settings enough to feel comfortable that I can fully protect it. So, I typically disable it on my OpenWRT devices. Here's the magic shell commands that turn it off for the lan and wan interfaces (if you have other interfaces, you'll want to adapt this for yourself).
uci set 'network.lan.ipv6=off' uci set 'network.wan.ipv6=off' uci set 'dhcp.lan.dhcpv6=disabled' /etc/init.d/odhcpd disable
Don't forget to save the settings!