I posted this in another thread.
I’ve been travelling the forums and the net looking for the same solution and I finally came up with a combo that’s easy to use and works great.
First I created a new user role using the Role Manager plugin (you can disable the plugin after you’ve used it and the roles remain).
Then I bound individual users within the new role to specific categories made only for them using the Bind User to Category plugin.
Finally, I hid options and selections (including the category selection option) from those users with the Adminimize plugin. This last plugin was the key that I had been searching for and couldn’t find.
Now, userA can only post to the userA category and it’s by default. userB can only post to the userB category by default, etc.
Hope this helps.