If your router supports it, you could either redirect or at least block outbound DNS port 53 traffic for all devices but Pi-hole (though chances are that wouldn't prevent DNS requests through your router's IPv6 address).
If your router doesn't support controlling DNS traffic, you'd have to manually configure a DNS IPv6 server address on each and every client in your network.
Not all clients may allow you to do so - most smartphones, tablets or IoT devices usually don't.
If you are not dependent on IPv6, you could also consider switching off IPv6 on your router completely - provided your router supports it.