WEll, if net send is not working, and you have admin rights, there are ways to shut a machine down remotely. If you dont know them, take the time to research. My telling you that much should give you a inkling as to what to do. I dont like the sound of the thread. If you know the IP address and no one is anwering a net send... do they know HOW to reply to a net send? maybe have them call you immediately after recieving it?
also maybe tell them if tyhey dont you will shut the PC down...
As far as shutting it down, the only hint Im gonna give you is google. Somewhere out there youll find the RESOURCEs you need... they are in some KIT I believe.

EDIT: also if you shut the PC down and its DHCP, chances are youll get the same IP when it comes right back up...but there is the chance you wont. Bear that in mind... mac address dont change so...