There are plenty of solutions on the IT side, such what sstoppel mentioned, or if possible, adding the PC to the LAN and using any of the myriad of remote monitoring programs to keep an eye on it.
However, is there a human resources solution to the problem? IE, implementing a metric to determine the productivity of the employee? If they are getting their work done and there isn't more to due, you may not have a problem at all. If they are getting their work done and then killing time, they may need addition responsibilities.