I am not sure if my solution will serve you.
In my discussions, I use Ratings. Then, I set an ending Rating date to correspond with when the discussions have to be made by, for points/credit. When the date passes, any student making a post no longer allows me to Rate their post, so they cannot get credit for late posts. So either you, or even students can post after the due date, but the students late posts cannot be rated.
Consider visiting the Moodle Tracker system and voting for MDL-31355. It is targeted at providing some improved "date" features to forums.