That's a rather ambiguous question. First of all, can you force them to turn on their PC's?
Only supply them with a ridiculously small internal disk. If they want to store any "real" data then they'll have to use a drive mapped somewhere. I can't think of a way to forbid any MS application from storing on particular drives, like C:.
What problem are you trying to solve? That they store their data on the server to ensure that it gets backed up? Maybe it's time to look at something like Tivoli Storage Manager, or TSM?