Hello Mak,
the first issue you have is to describe the applications. While at first blocking specific TCP/UDP ports seems the best choice, you will learn, that those applications can easily be reconfigured to use different ports.
What you can try is still the access-list approach to block the majority of traffic. You should try to find out which ports these applications are using in your network.
Another option is to use a router with NBAR and "match protocol gnutella" and the like and apply a service-policy to drop this traffic.
