Why do you leave your wifi open?
I handle one of those types of cases every week (I work for an ISP). Here's what typically happens:
1) We receive a nastygram from a production company lawyer.
2) We match the IP address and computer ID to a customer and forward the message.
3) A followup nastygram is sent to us, demanding action because there has been no response.
4) We disconnect their service to force them to call us.
5) They always say something like "someone hacked me", "A virus did it", "Someone got on my wifi", etc. Not once have I ever had someone say "Yeah, I did it. I'll delete the content (which would make the problem go away)."
6) We go round and round, and sometimes the police show up to subpoena our records.
7)After that I imagine a lawsuit takes place.
So...if you actually did download something you weren't supposed to, simply say something like "I downloaded "whatever" and the website I got it from presented it like it was free content. Since you have informed me I obtained this illegally, I have removed any and all copies from my computer and will not visit this website again. Thank you for informing me."