I cam accross this video http://www.momversation.com/video/your-child-obsessed-video-games
where they discuss their kid's video games habits. My son used to have this problem and it seems that nothing matters anymore but their stupid computer games. I have to be firm and be authoritative to my son until he learn and rarely plays anymore. Study first before those stupid games.
But if your kid is really addicted to it, the best cure is, buy games that are easy and boring. I'm sure that it's just a matter of time and your kid will get bored playing those games and do something else.