The discussion locking option for forums is different than setting a due date for participation—

If you enable the locking function, individual discussion threads will close down after a set period of inactivity—the forum as a whole will remain open to continued participation, but any thread that hasn't had a response made to it after that time will be shut off from new posts. The image at right shows what students would see once a discussion thread closes.

We don't recommend enabling this function unless you have a compelling need.