[CentOS] Blocking radio streaming

Hi,

I'm the system's administrator at my company.
All servers are CentOS :)

I've been noticing that many people spend the whole day hearing to 
online radios.

How can i block the streaming ?

It's HTTP and i think it's through port 80.

Any help would be appreciated.

Warm Regards,
Mário Gamito
PS: No good blocking the radio sites, they'll discover others.