Okay, this is a possible solution to combat 'necro-posting'.
After x amount of days*, a certain thread becomes 'locked', if you will, and refrains anybody from re-posting on it. There is an exception for the actual owner of the thread, and members of staff.
(*Without anybody posting on the thread.)
I understand that it will cause soom people to try and 'bump' up threads more often, but I could see it as a better solution than the one we have currently.