I wonder if it is possible, when someone clicks on the reply button, to check the date of the last post, and then have a little pop up that says:
This thread is over 6 months old. Are you sure you want to reply?
The pop-up wouldn’t show up if the last reply was less old than some cut off value, and it wouldn’t prevent someone from posting. It would just give a heads-up to someone who may have found the thread via a search or a link and not have noticed the dates.