The classic problem, someone connects to your wi-fi, and tries to download a movie, then you get a nasty email from your ISP making you look like an organized crime ring lord.
what you looking for is a traffic filtering solution
I used to have a complicated routing table blocking every known p2p site, known to man. Then I switched to creating my own DansGuardian/pfsense implementation. After moving, I lost all the configs, and had to settle for OpenDNS, which is a very good solution, direct solution.
The setup is easy.
1- change your dns servers on your broadband network, not local network.
Your gateway IP is on the router, with the password. A common one is http://192.168.1.1/
go into Network connections,
and point your DNS servers to OpenDNS servers
2- setup your openDNS account. go to OpenDNS.com the setup is as easy as creating an email address.
you'll be asked to provide your public IP. A simple google search "what's my ip" will give you that information
OpenDNS now can configure filters on your behalf. Summary: your DNS points to OpenDNS, which in returns knows yout ip, and filters content based on your requests. It's that simple.
you can select pre-configured filters, or select your own custom ones.
and on a good day, you block, your targets.