Just posted a similar response in this other thread:
Basically, we decided that the "captcha"-esque idea was the best way to go. Some way of making the user confirm that they know it's an old post before they reply to an old thread.
Also, if you do mark the last reply as spam and it actually gets removed by someone who has permission to do so, I believe it does bury the thread again. I'll ask sampy though to make sure.