Anywhere in the IPv6 Address you should find the MAC of your client. But there are implementations (IPv6 Addons) to obfuscate this.
I do think the client getting it not from your Flint, it is more a SLAAC (RFC 4862 - IPv6 Stateless Address Autoconfiguration) thing.
The only way to avoid this is to deactivate IPv6 on the Client.
IPv6 will be a great and worthy successor of IPv6. But it will be different.
Please don’t underestimate the usefulness, just because it is different from ‘the known’.