You need to have a written policy clearly defining the expectations with respect to gaming, net surfing, etc. Enforce it. If you catch someone, give them a verbal warning. If you catch them again, give them a written warning. If you catch them a 3rd time, give them the can.
The policy, however, should simply state that you could be axed for playing games. If people know they have a 3-strikes rule, they will likely take advantage of at least one of the strikes.
Finally, a slacker is a slacker. If you forcibly block them from doing one thing, they'll waste time another way. If it's not gaming, it's chat. If it's not chat, it's solitaire. If it's not solitaire, it's frequent coffee breaks, and if it's not coffee breaks, it's bathroom breaks.
A good worker will get his stuff done. A bad worker will not. Workplace distractions should be viewed as a management problem, not an IT problem.