Just delete the game without making a move, by clicking the trash bin icon next to the game on your MyGames page. People don't usually get offended, it's not always convenient to take on new games and there's nothing to stop you sending a PM to explain why, if it really bothers you. Many of us like to challenge new arrivals to a game just to help them get a few games under their belt, but we usually have too many games already so won't get upset over a deleted game.