There's already a message that comes up pointing out that it's an old thread and asking if the person really wants to bump it:
Old Thread Warning
This Thread is more than XXX days old. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
If you feel it is absolutely necessary to make a new reply, you can still do so.
[check]I am aware that this Thread is rather old but I still want to make a reply.
Not sure what the minimum elapsed time is to trigger this, but if that doesn't do it...
Actually, I think new members should be prevented from posting at all
until they reach some minimum post count.
Oh, no, wait... there's something wrong with that.