How to block Skype in my domain network

Hai Friends ,

I think u must be knowing about the Skype Its a free software to make internet call & for chating .But my Boss need to block the users who r using this software .I got a Domain set up in my Factory & i too have a Firewall running .
I need to block the users who R running the Skypee.exe.SO friends please help me by giving ur innovative ideas.

Network Type :Domain running on Windows 2000

With a Firewall running on it .