The "prevent article author to vote" function seems erratic, and I'm not sure if I'm doing something wrong, or if this may be related to the recent rewritten version of the plugin.
I was installing it on WPMU 2.9.1, and checked the boxes next to "Prevent article author to vote" and "Prevent comment author to vote", saved the changes, and noticed that authors can still vote on their own posts (thumbs). This was using WPMU 2.9.1. I tried using the Default theme and had the same problem. GD Star Rating does successfully prevent comment authors from voting on their own comments.
Next, I did a fresh install of WP 2.9.1 (not WPMU), and again checked the "prevent author to vote" boxes and saved it. This time, the author is successfully prevented from voting using thumbs. However, the author can still vote using stars. I didn't change any of the other default settings. I was using an Admin user to test this. I tried using an author user and I had the exact same problem: he is prevented from voting on thumbs, but not on stars.
I'm not sure if this is related to the recent upgrade of the plugin (7 days ago) or maybe I am doing something wrong. However, I've pored over the settings and I don't see what I could be doing wrong...