Limiting Category posting by Author

My overarching goal is to have mutiple authors, each with their own blog, tied to their own theme, on their own subdomain.

It seems that my best shot at this is by using authors, and categories .

At this point is there a way to create a category for author Bob called Bob, and when Bob logs in he can only post to his own category? It appears that a new post defaults to the default category, and Bob is able to put his post in any of the categories, including Joe and Schmoe’s.

Has anyone done what I am trying to do/