Display only one category on Author Archive

You can achieve this in several ways.
One is modifying the code to the author.php in your theme. If you don’t have an author.php, copy the archives.php rename the copy, author.php
and modify that one.

The the Loop…
http://codex.wordpress.org/The_Loop

You will want to filter the category you want.
You can also use a plugin if you want to be able to add different categories later, and not have to be modifying the code.

I’m using role scoper

This lets you add Restrictions and Roles… one of those restrictions, is you can modify your categories and allow or deny different types of users to view and/or edit them. make them visible to all public or just to specific roles, etc…

So there you can just make that category which you want to appear the only visible one, and that should do it 😀

Good luck, let me know how it goes!