I don't know how your network is set up at work but a couple of things you could do are

1. If you can't find the games they are executing, rebuild the workstations and lock your network down so that they don't have rights to download/install software. It's dangerous to let ordinary user have the ability to do this anyway.

and 2. Like Zero said scan the registry and get rid of things that aren't supposed to be there.

Hope this helps.