You can't un-buy games.
You can only un-rate a game.
Use collections to organize your library, instead of the Things you own.
I for one would also welcome an option to at least hide purchases. You can display the games you own publicy, but maybe you do not want to show the world the purchases you were unhappy about. There is collections, but displaying it on your profile is bragging and bragging works better, if the list has the official sign that you supported a game.