Well you could also do this, buy a external hdd case (newegg.com) and than rip out the hdd, and put it in the case, than plug the external hdd to a computer that is almost always on and done. It will save lots of power!
There is a way to shut down computers outside of the network. You will first need to open a backdoor on their computer and than get access and run the shutdown (Illegal)
A legal way is what Lukasz said, shutdown computers in network.