To manually stop students from posting replies to discussion posts, you can use the Close for Comments option.
Notes:
In Course Navigation, click the Discussions link.
To close a discussion from the index page, locate the discussion. Click the Options icon [1] and click the Close for Comments link [2].
To open a discussion for comments, locate the Closed for Comments section. Locate the discussion and click the Options icon [1], then click the Open for Comments link [2].
To close comments within an individual discussion, click the title of the discussion.
Click the Options icon [1], then click the Close for Comments link [2].
To open a discussion for comments, click the Options icon [1] and click the Open for Comments link [2].