Thanks for the replies. It looks like I have a few options to investigate.
Whilst I have no reason to distrust my lodgers I am aware that the filesharing lawyers tend to hold the bill payer accountable for all use of the internet connection, and with that in mind my guests may be inclined to download a few MP3s or movies. I am just keen to minimize the risk of any legal bother.
In the absence of blocking, would it be possible to log traffic through the router so as to provide some evidence, if necessary.
Thanks for all the helpful suggestions.