Here's one way that's probably not foolproof, but would at least reduce the possibility of someone getting away with it.
- Set up an account at http://www.opendns.com They're free.
- Choose settings and create a network & give it a name to be your home network.
- Pick that network in the "Settings For" drop-down
- Choose "Web Content Filtering" on the left.
- Select the "Custom" radio button at the bottom of the list.
- In the large array of radio buttons which then appears, make sure you have "P2P/File sharing" checked.
- Configure your router to use OpenDNS's servers for DNS.
Now, on some routers you can configure them to log in to your OpenDNS account so they know who you are when your router gets its IP from your ISP. On others you may have to run one of the OpenDNS clients to identify your network to the servers at connection time. See the help at the OpenDNS site.
This won't stop a determined user. (For example, they could bypass the DNS address you're providing them and use their own). But it will at least show that you made an effort if someone came and accused you of something.
Disclosure: I do not work for OpenDNS.