crazy_dave: You should be able to hover over the number of stars and it will change the stars to that be that number more or less (at least that's its behavior under >My account>Game info. However, I think once you have given a rating, I don't *think* there is a system to completely remove your rating from a game.
Lorfean: Seems to me this can be done either by left-clicking on the game in your shelf and adjusting the rating on the "Game info" tab, or by going to the "User reviews" section of the game's gamecard and adjusting it there.
Oh, okay, thanks. I didn't check under "Game Info", I just checked the games list under "My account" (the alternative to the shelf display). In that list strangely enough you can add stars but not remove them ...