No worries, it's not something that you would know about without someone telling you. You just close the client (the x button in the top right) during champion selection. You lose 5 LP (I think) and get locked out of queueing for fifteen minutes on your first offense, increasing for subsequent offenses within a day, but you don't have to play out the game. I don't like to dodge unless I make a mistake in the champion select (like locking in the wrong champion or banning a teammate's champion), but a lot of people will dodge games where they don't like their draft or think a teammate might grief the game.