I think the best way to handle this is to explain in a direct assertive manner how his complaining is bringing you down. Furthermore it is not healthy and in no way will help him win nor bowl better or ever be agreeable to anyone including him/her. Direct is best but all too often we're told things in an off hand way and given a riddle to solve. Pepple are so afraid of confronting issues these day,directly.

I like the nerf gun approach as well.