Or actually monitor them. Their are some programs, like Track-It, which can audit your networks hardware and software, along with tons of other features -- license tracking, help desk dockets, remote access. Do an audit, see what software is loaded, and nail the offenders.

If that doesn't work, use policies to prevent installation or introduction of files or folders onto the machine. And since they have no net access, you don't have to worry about net games.

I would like to note that if they are only playing during lunch or during breaks, I personally wouldn't care. It's a good destresser, and if it's ok with the heads, let 'em play. If it blends into their productivity, nip it in the bud. We used to play UT during lunch -- nothing gets rid of stress better than fragging you opponent. But we only played during our lunch hour, no more.