In response to a recent flurry of complaints regarding the posting to obsolete threads, I would like to suggest a possible solution.

Rather than retype it, here is my response given in the Addicts forum
I would like to assume that people responding to these old threads are simply unaware that they are so old. The flashing date simply does not stand out enough to be noticeable. Many of these replies would be avoided if people could be made more aware of the state of the thread; therefore limiting those responses, hopefully, to the more useful ones.

To that end, I would recommend a message, instead of simply a flashing date. Something along the lines of "This thread is more than three months old. Please do not reply to this thread unless you are making a correction, asking for clarification, or answering an as yet unanswered question." This could be in the form of a text message on the line beneath the date, a javascript popup, or an additional page requesting verification before posting.

I am against a blanket ban. It would prevent questions and answers from being indexed on the same page, and thus make the forums harder to search than they already are, as well as adding additional load to the servers.

Unless a proactive measure is taken to minimize the problem (an explanatory message), and not just a reactive measure (in the form of PM's or AP's), any and all efforts made to minimize the problem will be wasted. Nobody reads the welcome email anyway, much less search the page for the posting date
Sorry, no time to paraphrase it for Suggestions.. I must be off to bed now.

Another request you's to deal with. Have fun with it.

Kudos to Katja for getting my brain in gear on this.