To manually stop students from posting replies to discussion posts, use the close for comments link. You can also pin discussions to the closed for comments section on the discussion index page.
Note: You can automatically set your post to close for comments on a specific date by creating discussion availability dates.
Click the Discussions link on the Course Navigation.
Locate the three-dot icon and click the Close for Comments link.
Note: For Graded Discussions, the Close for Comments option will not appear until after the Discussion due date.
Verify the discussion is closed for comments by locating the selected discussion topic is moved to the Closed for comments table.