protecting categories from posting

Hi,
I’ve only just set up a wordpress blog so I’m new to this.

I am working on a blog with a number of people posting to it.
Is it possible to only let one user or users above a certain level post to a category?
Example: Only ‘admin’ or users of level 10 could post to ‘News’ Category?

Is this possible already, or could it be done easily with some php code? (nothing too complicated, please)

Thanks for your help,
Neil