I recall seeing discussions about this issue, closing discussion forums, over the last several years. I know that I have had discussions about Forum dates (see MDL-31355, and vote for it.) And I can appreciate Howard's comment that "forum is not really intended as (primarily) an activity for submitting work" but I don't agree because, in practice, many instructors use them for this purpose. I have been using forums forever as an assigned activity, just like a quiz or assignment, and I don't think that I am alone in this approach.
However, I am also one who doesn't think forums should be closed, although I do see that many would want to do this. I like to encourage my student to always post ideas, even if a topic has expired. Realistically, students seldom do this because they are too busy posting in un-due discussions where they can get "points."
I do like using the Ratings "To" date as a way of letting me know that a student's post was late. In this manner, I am not allowed to rate a post after this date. Yep, I find this uniquely helpful.
So an Availability or Timing block with the following could be an approach:
Open the Forum (date when posts can begin, and rating can begin)
Due Date (date when the forum posts are due, this goes into the calendar)
Rating Date (the date when forum rating expires)
Close Date (the date when no more posts can be made, what you describe as "lock" date)
The "Restrict ratings to items with dates between Open and Rating dates" can still be there.