I think I saw this on an episode of the Brady Bunch, where the kids were always on the phone, so Mike got a payphone, but then the kids were all bothering their parents for change, so Alice had to start turning tricks, or something liek that, my mammaries are a little fuzzy this morning...
Anyways.. I've seen on Happs site coin timers, like you would see on a batting cage/pitching machine where you pop a quarter in the slot, it ticks away for a minute or whatever and then DING no more fastballs.. I would assume that the timing can be changed to set it for 15 minutes or however long you need.
If you're just looking to secure the PC, there's all number of keylock switches, bios passwords, heck, I bet there's even a way(if you're running XP or 2000) to set the maximum login length to a set amount of time
There are also a few Rube Goldberg-esque contraptions that come to mind...
