I've learned to decline gifts if it's a game I'm unlikely to be interested in. Puzzle games, for instance, or platformers. If the gifter thinks I'd really like it, they can show me the game in person, but if it was just "hey, I've got extra copies", I'll let them know they'd be better off sending them to someone else if it's something well outside my interest zone.
But I do have a lot of unplayed games that I bought on the cheap and then went back to Civ, Europa Universalis, or one of my other usual suspects.