Give author his own category

Hi,

I would like to add an author that can only post in a certain category. I would like them to ‘guest-blog’ on my website. I already found out how to use a different theme for their category, but now I want to know how I make sure they can’t post in another category than their own.
I’ve got an account called “guestblogger” and I want to make sure they automatically post in the category “guestblog”. Is there a plugin that does this for WP 2.7?

Thanks in advance!