Deny, Deny, Deny. Don't respond to any letter. Had this happen years ago from Road Runner before they became Comcast. They had listings of my directories and asked me to respond. I didn't. Another time my account with Comcast was shut off. I called and they said there was activity that looked like my account had been hacked, and for me to be careful. I played along acting dumb and they gave me a warning.
Machine, were you running the tracker 24/7? Today, I don't run the tracker 24/7, only when I need to download something. I built up a huge ratio years ago from seeding UFC torrents. I use the "zap" thing to clear hit and runs. I log into my account on the web to keep it active.