I think the more appropriate question is how to avoid re-opening a 6+ month old thread that was "time specific" around a particular event......
You are asking the impossible. I was thinking about a good feature for the board. In the same area you set to automatically subscribe to threads you reply to have an option to automatically unsubscribe you from threads with no activity for over a set period of time. So if there is no activity for say 3 months, you consider the thread dead and automatically unsubscribe from it. This way, even of someone posts to an old thread, you won't get an email telling you there is a new post in the dead thread.
Makes me wonder though... did Melanie get a job as a lawyer?