Block all IPv6 traffic for one client

I am trying to find way how to block all IPv6 traffic for one specific client. Its stupid client, so I cannot to block it directly on it (it connect automatically to IPv6 if available).

I am able to identifi this client by mac adress or IP. Can this be done for example by some firewall rule?

http://192.168.1.1/cgi-bin/luci/admin/network/firewall/rules