Oh my... so many misunderstandings in a single message.
Free.fr is the consumer branch of Proxad, one of the larger French ISPs. Enabling native IPv6 on a Free.fr ADSL line is a simple matter of clicking the right box in the router's configuration interface. (Actually, it's not native, but 6rd, but that's beside the point.)
Free.fr IPv6 addresses come from the 2a01:e00::/26 block, and there are tens of millions of customers in there. Seeing many connexions from that block is perfectly normal.
> Still way too aggressive incoming UDP anyhow!
It's most probably µTorrent implementations attempting µTP connexions. And it's not a single guy, as you insist on believing, it's the whole customer base of Free.fr.
--jch