you need a sharp lawyer who won't rip you off. hire one. free advice is worth every penny you pay for it. however, IN MY OPINION, if you had a sharp lawyer who wouldn't rip you off, such a lawyer would tell you how to translate "fuck off, this has nothing to do with me" into lawyer-speak.
the translation would (IN MY OPINION) be brief, clear, and non-argumentative. whatever you do, DON'T be upset by anything they say to you. provoking rash reactions is just a tactic they employ.
if I were a lawyer, AND I AM NOT, I would tell you to write a letter that looked more or less like this:
"Dear Whoever,
I read your letter with interest, and noted your request that I shut down XYZ Site. However, I am unable to cease operating XYZ, because I do not operate it, and have never operated it at any time. In fact, I have absolutely no connection to XYZ, have never had any connection to it, and am unable to help you. Good luck, and have a nice day."
Again I AM NOT A LAWYER, but that's really all you need to say. You put that in the mail, certified of course, with receipt, so you can prove they got it, and you forget this ever happened. Any aggressive, offensive noises they make other than "here is your court date" are just NOISES. Ignore them.
In the unlikely event they get you in front of a judge, the judge will hear your simple defense - "nothing to do with me" - and ask them if they have any proof of it being anything to do with you. Since they don't, it's a short conversation, and you go home.
If you find yourself saying anything further than what I just described -- "not me, got your letter, nothing I can do" -- STOP IMMEDIATELY and either hire a lawyer or shut the fuck up.
speaking of hiring lawyers, again, disclaimer: I am not a lawyer, this is not legal advice, grain of salt, your mileage may vary. seriously just hire a fucking lawyer because panicking, freaking out, and/or writing 10,000 words about this is just completely wasted energy.