Under Web Protection -> Application Control enable Network Visibility. Create a new Application control rule, and in "Control these Applications" select "Streaming Media" category and choose apps that you want to block.
For example, this would be a rule that blocks Youtube streaming with "Youtube" and "Google Video" application definitions.
web proxy...application control is one way. Also stream youtube and in the flow control you can see what is going on in real time and block it from there.