How to have admin-only category on multi-user blog?

Let’s say I have a blog where all registered users can post and comment. On this blog, how can these registered users be not allowed to post under a specific category (say “site announcements”)? I mean, only have admin post in this category. All registered users, however, can comment on posts in this category. I can wrestle with code if someone lets me know what needs to be changed to get this to work. Any pointers will be appreciated.