I am so with you. It is so frustrating!! Here's what we have done in our house: we used the system control device to limit the play time. No more policing or nagging... when your child gets to the time limit that you set, the system just turns off! Not sure what gaming system you have but most have a built in device that lets you limit your kid's time. Here's a link to the one for xbox that we use: http://www.xbox.com/en-US/support/familysettings/console/xbox360/familysettings-familytimer.htm
Frustrates the heck out of our son - but it works : ) If this is not your system I would suggest googling the name of your system + how to limit my child's time.
This is a big issue, my two children ages 14 and 15 want to play four hours a day...it is out of control. I have limited it to 1 hour a day during school days and two hours a day on weekends. Trying to get them to do some other activity is a constant battle. I have taken it away as a punishment for grades and have had to go to lengths of removing the CPU or wireless. Help