The following steps will disable IPv6 on your OpenWrt router. All the steps are performed via the command line.
You can perform them in the console of the router but the preferred way is via SSH.
uci set 'network.lan.ipv6=0' uci set 'network.wan.ipv6=0' uci set 'dhcp.lan.dhcpv6=disabled' /etc/init.d/odhcpd disable uci commit
uci -q delete dhcp.lan.dhcpv6 uci -q delete dhcp.lan.ra uci commit dhcp /etc/init.d/odhcpd restart
uci set network.lan.delegate="0" uci commit network /etc/init.d/network restart
/etc/init.d/odhcpd disable /etc/init.d/odhcpd stop
uci -q delete network.globals.ula_prefix uci commit network /etc/init.d/network restart