Saberwolf_Prime: If you have an unwanted game or an extra copy on your account (GOG or otherwise), you should contact support and let them sort it out for you.
Sticking a big red button to games on your account seems dangerously easy to abuse, make a mistake, and not to mention being a more tempting target:
"Some people just want to watch the world burn."
-Alfred
Edit: ok, it may not be that easy, but it is no less prone to abuse.
How can you make a mistake? You need to open a browser, go to Steam, go to Support, click on Games, Software, etc., search the game, click on it, click on "I want to permanently delete this game", see the dialog where you are informed what you're about to do and you need to click again "Ok, remove the listed games from my account permanently". A lot of clicking in order to activate the "red button".
Even few days ago i saw people on Reddit requesting the removal of some games from their account. If it's something that people want, it should be there.