"As you have found out, copyright holders hire anti-p2p orgs to monitor torrents with their content. They join the swarms, copy the IPs from the peer list, and send out letters to the ISPs they belong to.
What should you do now?
You should not respond to the letter if at all possible, and if you must respond to it (for example, some ISPs have begun disabling internet access on just the first offense until the customer contacts them) you should not admit to anything. Plead ignorance. Tell them for all you know it was a family member, a friend, neighbor etc. Ask them if it's possible that they made a mistake. None of those are valid excuses as the customer can be held to account for any activity on their account, but they are better used than any admission that you did download the content in question.
Then quit downloading anything at all at least for a while.
If and when you do resume downloading, stay away from newer and more popular content. All copyrighted content can be monitored and earn you another letter like yours (or get your service disconnected), not just newer stuff, but the newest stuff, ie: cams, telesyncs, screeners, r5s, newly released DVDs/blu-rays, popular games, television shows, etc, are much more likely to be monitored than older stuff will.
Don't be duped into a false sense of security by any app that claims to offer you some protection from being detected. IP blocklists like PG2 & Blutack etc are useless."