We too just received a legal notice that shows that a guest illegally used our internet. Airbnb suggested adding house rules. I also contacted the guest, am awaiting their reply, and documented the name of the guest.
My internet service provider suggests settings on a router so I am researching how best to block illegal downloads (torrents) and sharing of copyrighted material. I will look more closely at the suggestions in this topic to see if I can take care of this myself as I am a little technically challenged.
Add to that that our rental is a three hour drive and it becomes frustrating. Last month I had to travel to the house to use the TV remote to BLOCK guests from upgrading my service. Alas, my list of house rules just seems to grow longer and longer.