I have writen a small plug-in called « Restrict Categories » for the great WordPress 2.3!
This is a category user filtering. Just check the category you want him not to use. Simple.
Download version restricat 0.3.2 (Please report bugs! Beware, only in french for the moment)
- Note: if you want articles from a sub-category to be restrited, be sure that you have checked the parent category also in the list when writing.