Maybe they knew about your client by doing a whois check with his/her domain and then visited your site by following the link in the nameserver. They may have got your personal information through another check (I have just done checking for your site's information at
www.checkdomain.com and what I got was pretty detailed) since I could see your name, domain nameservers, city,etc...
I don't know much about cities, locations,etc... so I am not sure if what mentioned is in China or not :P but I guess that's the way they have got your personal data. The best solution for now is to add all of them into your ignore list (I usually do this when an unknown person adds me and then talks about crazy stuffs like that).
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