Update (2012/05/30): There is now a plugin for limiting post-navigation to the same author and category. The method in the article remains for informational purposes only.
In this DigWP post, guest author David Powell explains how to limit WordPress’ default post-navigation to include posts only from the same author and category as the currently viewed post. This method is effective, but involves making modifications to core WordPress files, something you should only do as a last resort, and if you know what you are doing. If you’re comfortable with that, read on to learn how it’s done..