You can enable logging on the router, and see which ports are being used. One of many resources for port info is at
Once you determine that a given port is not used for appropriate business purposes, you can block it. Since many of the P2P services can use a wide variety of ports, you will be playing a cat-and-mouse game. From my perspective, this is more of a policy issue than a technical one. Does your organization have a clear and well-known appropriate use policy? That would be a good place to start, with your management taking ownership of it as a business issue.