Meh, I’d probably ignore it. If it’s real, you’ll probably get something, in writing, from a lawyer asking you to take it down. If that happens, then I’d worry about taking it down, otherwise I’d move on.
If you want to take it down anyways, and you can do so easily, go ahead. Sure ‘they won’, but who cares. FTR, unlike here, some message board posts are editable forever. I post to some boards where I can go back years later and change or delete a post. Of course, that also bumps it up to the top.
But, in the future, you might not want to make those kind of posts in a place that can so easily be connected to your real life persona. If you can figure out how they connected that you on that board to your phone number, you might want to see about breaking that connection, if it’s possible.
(I know, with enough digging, it’s possible to find me from what I’ve posted here, so I do try to not post anything that would anger anyone to the point of tracking me down. If I could do it again, there’s a few things I might change to make myself more anonymous, but I’m okay with thing being the way they are).
Having said all that, if the person calls back, you could either just delete it, or if you want to stand your ground say 'could you please send me something on your letter head and I’ll be happy to delete the posts, otherwise please stop contacting me" and hangup. You could even take it a step further (but this might make things worse) and say, bluffing, “this call is being recording and I’m going to send it to the TV station that did the interview along with a link to that thread if you don’t stop contacting me”.
Also, you said you don’t even know what you posted, you might want to see what it is. If it’s nothing, that you really can ignore it, they might have just contacted everyone they can find in that thread. If you posted something that wasn’t true (and it sounds like it might turn out to be the case) you also might want to take it down, just in case. But, like I said, you could just wait until you get something on paper. I’d be surprised if some random internet poster got hit with a defamation post. If that was common, politicians/CEOs/business owners would have a field day suing people.
TL;DR, if the threat is real, you’ll get something in writing, from a real lawyer before they lay out real money to sue you…probably.