IMHO you or your company, whatever, have a management problem which you are trying to solve with technical means.
BTW by using DHCP you can tell the hosts which DNS server to use.
If you configure a DNS server which returns 127.0.0.1 addresses for rapidshare.com, and your DHCP server hands out this nameserver's IP address to the DHCP clients, you will have reached your initial goal. Rather easy to circumvent for smart folks, but diffucult for not-so-smart folks.
Get a book like Craig Hunt's "TCP/IP Network Administration' and you can configure such a nameserver within an hour or so.