A simpler approach would be to SSH into your OpenWrt router and execute the following commands:

uci set 'network.lan.ipv6=off'
uci set 'network.wan.ipv6=off'
uci set 'dhcp.lan.dhcpv6=disabled'
/etc/init.d/odhcpd disable
uci commit

The first command disables the IPv6 LAN interface, the second the IPv6 WAN interface, the third and fourth disables the DHCPv6 Server and uci commit commits the changes.

UPDATE

It seems my information has not aged well... The settings are a bit different and these are additional bits. The information should now be up to date for 2021.

The syntax got changed a bit:

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

and then disable RA and DHCPv6 so no IPv6 IPs are handed out:

uci -q delete dhcp.lan.dhcpv6
uci -q delete dhcp.lan.ra
uci commit dhcp
/etc/init.d/odhcpd restart

You can now disable the LAN delegation:

uci set network.lan.delegate="0"
uci commit network
/etc/init.d/network restart

You might as well disable odhcpd:

/etc/init.d/odhcpd disable
/etc/init.d/odhcpd stop

And finally you can delete the IPv6 ULA Prefix:

uci -q delete network.globals.ula_prefix
uci commit network
/etc/init.d/network restart

UPDATE 2

To quote @Kurt Fitzner:

Use ifconfig to show all the network devices and note which ones have IPv6 addresses. Then navigate through the graphical OpenWRT interface to Network->Interfaces, and select the Devices tab. For each device that had an IPv6 address, select "configure" for that device and uncheck "enable IPv6".

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