How to stop them? HELP ME PLEASE…

Hi! I am managing a network of 200 workstations running on windows 2003 servers in lab. I have a problem, I have provided some material to study in form of word and PDF documents these files are residing on one of the member server. Now what happen is users copy these files in their own logins and it causes two problems;
1-low bandwidth on network due to heavy copy traffic (there are more than 800 users that work at different hours)
2-it almost eat an ample amount of storage.
I want to prevent these users from being able to copy these certain files from server and paste them in their logins. But i also want to allow them to create new files in their logins?
ANY HELP….