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.
Close For Comments
Locate the Settings 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 This topic is closed for comments indicator.
Re-Open for Comments
If you want to re-open your discussion, locate the Settings icon and click the Open for Comments link to re-open the discussion for comments.