Some firewalls will block
desired extensions and content types, like .wmv, .avi, etc...
Read
this. Also
this.
I've read that most streaming sites use port 80. You could set up a proxy server. Block port 80 and use 8080 for instance.
You could also use the Group Policy editor:
User Configuration > Administrative Templates > Windows Components > Windows Media Player > Networking.
Now choose the Streaming Media Protocols element (which will probably
be set to "Not Configured") and set it to Disabled. You might also
want to "Hide Network Tab".
Can't company management control 10 employees?!? A strict policy would be much easier to enforce for 10 than for 100 employees. However, happy employees are normally more productive...