It may be possible, but you may find that you won't entirely block it. A user can configure real-time player to use http as the transport, so unless you block port 80, the audio stream can still get through.
I know at one time real player used one tcp port as a control port and the other ports were negotiated dynamically. I am not sure if this is still true, nor do I know how ms media player works. If a control port is used, it makes your job easier - just block access to the control port.
If you know that there are sites that offer streaming that you don't want your users to reach, you can try filtering by site, but it can lead to this: what if the same site (ip address) offers web and video services, and you want to reach the web service.
If I come across anything interesting, I'll forward them to you. Just let me know how.
