Oh, I think (and I'm not sure as I've not done it myself) you just change the title to say that the offer is closed. Does that make sense? I'll look into it some more, but that's how I take it. Good luck!
ETA: I see that you did change your title on the thread to which I think you're referring. All of the other "Closed" threads on that board did just the same thing -- changed the title to read "closed: ___", but the thread is still technically open (can reply to it, etc). It seems you've done what they were asking you to!