How do you cancel a thread that you have started if it no longer applies?
I usually send a PM to our volunteer webmaster Chris Brancaccio requesting that he delete the thread.
Here in the discussion part, just write a message that the question no longer applies and hopefully the thread will drop down into oblivion if noone adds anything to the discussion. Would work in the classified section too - write a message in the thread that whatever item the thread is about is sold or no longer wanted and let the thread drop as other threads are updated or added. There are no older threads than october 2014 in the classifieds, perhaps they are deleted after some months? (Can't remember when the new classifieds section was added, but guess it was earlier than october?)
If something seriously offensive is written, Chris Brancaccio, our valued moderator can delete the post.