How do I allow or disallow announcement replies in a course?
By default, comments are disabled for announcements. When you create an announcement, you can allow comments on that announcement by selecting the Allow users to comment checkbox in the announcement options. Additionally, if an announcement allows users to comment, you can require students to post to the announcement before seeing other replies. The Allow users to comment option is persistent, meaning the option you select when creating or editing an announcement will carry over when you create a new announcement in the course.
You can also allow or disallow comments after an announcement is posted.
Note: If announcement comments are disabled for your course, comment options may not be available to you. You may be able to manage announcement replies in Course Settings. If this option is unavailable in Course Settings, it may have been locked by your Canvas administrator.
Allow Comments

Click the Allow users to comment checkbox [1]. You can also require students to reply to a post before seeing other replies [2].
The Allow users to comment option is persistent, meaning the option you select when creating or editing an announcement will carry over when you create a new announcement in the course.
Close in Announcements Page
To close an announcement for comments from the index page, locate the announcement. Click the Options icon [1] and click the Disallow Comments link [2].
To close more than one announcement for comments from the index page, click the checkbox next to each announcement [3] and click the Lock Reply button [4].
Open for Comments
To open an announcement for comments, locate the announcement and click the Options icon [1], then click the Allow Comments link [2].
To open more than one announcement for comments from the index page, click the checkbox next to each announcement [3] and click the Lock Reply button [4].
Close in Individual Announcement
To close comments within an individual announcement, click the name of the announcement.