Even if this were possible, I don’t see how such a rule could work. If you prevent topics titled “Forum Announcement,” there’s nothing stopping anyone from posting:
- Forum-Announcement
- F0rum Announcement
- Forum Annöuncement
- Forum Proclamation
OP doesn’t want to use a category or tags for this—but these are the tools already provided for handling “certain type of post.” You could use an Announcement category or (subcategory) with Create permission limited to the Staff group. A tag group assigned to Staff could also work, depending on how you want to highlight and refer to these topics.