This has surely been mentioned before. I've noticed an increase in the number of noobs who are resurrecting old threads. Based on what I did shortly after joining (before getting lightly slapped on the wrist), it's likely the posters just didn't notice the thread date. Apparently the purple banner isn't sufficient. How about a pop-up when the Reply textbox gets focus or when a character is typed with the same warning and the option to continue if something really earth shatteringly worthwhile is to be added to the conversation? I know we all hate pop-ups.
As an alternate suggestion, you know when you receive a PM, the reply box already has some greyed out text from the original message? How about preloading the reply box on old threads with something like
WE APPRECIATE YOUR DESIRE TO CONTRIBUTE BUT THIS POST IS MORE THAN 3 MONTHS OLD. PLEASE DO NOT REPLY UNLESS YOU HAVE SOMETHING REALLY VALUABLE TO ADD. POSTING IN OLD THREADS WITHOUT GOOD REASON MAY RESULT IN INFRACTIONS.
This text could be automatically deleted when the box gets focus or when the first keystroke is detected.