Can radio & video streaming be blocked?

At my work there are a number of staff who listen to the radio via the internet and this obvioulsy is a burden on our network. I have been asked to stop this streaming permanently but do not know how. Could someone point me in the right direction?

Our parent server is 2003 R2 with 3 other child servers(2003 R2 and 2000)connected to about 90 clients. Not all of them listen to the radio via the net but it is becoming a problem.