To manually stop students from posting replies to discussion posts, you can use the Close for Comments option.
Notes:
- You can automatically set your post to close for comments on a specific date by creating discussion availability dates.
- For graded discussions, the Close for Comments option will not appear until after the discussion due date.
Open Discussions

In Course Navigation, click the Discussions link.
Close from Discussions Page

To close a discussion from the index page, locate the discussion. Click the Settings icon [1] and click the Close for Comments link [2].
Open for Comments 
To open a discussion for comments, locate the Closed for Comments section. Locate the discussion and click the Settings icon [1], then click the Open for Comments link [2].
Close from Individual Discussion

To close comments within an individual discussion, click the title of the discussion.
Close for Comments

Click the Settings icon [1], then click the Close for Comments link [2].
Open for Comments

To open a discussion for comments, click the Settings icon [1] and click the Open for Comments link [2].
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