Greetings! Per this guide - How do I use the Announcements Index Page? - "A profile picture displaying the letter U in place of a user's profile picture indicates that an announcement was imported using a course copy, the Course Import Tool, or a blueprint course."
Announcements can't really be published or unpublished. If you don't want students to see the post, the most you can do is delay the posting of the announcement -- How do I delay posting an announcement until a specific date in a course?. If you're not sure when or if you want students to see the announcement, you'd want to set the delay until after the end of the course.
Hope this helps! Kona