You can disable comments for announcements and have that setting applied to every announcement throughout the entire course. However, you can also choose to close announcements on a per-announcement basis.
If you disable comments in your course and choose to remove the checkbox at a later date, the option only applies to new announcements. Any existing announcements will remain locked and must be opened to comments individually.
Note: Depending on your institution's preference, comments may already be disabled in the course.
Disable Comments on Announcements
Check the Disable comments on announcements checkbox.
Note: If the checkbox is grayed out, comments have already been disabled by your institution and cannot be changed.