You don't need human support for that; it's a self-service procedure.
You can only do it in the Steam launcher - select a game in your library, click Support and then "I want to permanently remove this game from my account".
The game will be removed from your library and the license entry will disappear from the
licenses log. The game/license will NOT however, be removed from your account. Once registered to an account - a license is bound to it forever. Even if you "permanently" remove a game with the above procedure, you can recover it (self-service). Even if you "overwrite" a previously "removed" license with a different one, you'll be able to check that it is still present - together with the new one.