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.