Just say no
Just write back and say no, it is not my responsibility to prevent other people from breaking the law.
You are no more liable for failing to prevent criminals breaking into your computer than you are if they break into your house.
Also you don't own an internet connection, you rent a connection, and that connection is frequently changed by the owners, your ISP.
Even if you allow that you did at some point use the IP address they claim you "owned" - the ease of falsifying such data makes it impossible to claim with any certainty that you were involved in file sharing.
As these lawyers appear to have no qualms in breaking extortion laws, you can hardly trust their "evidence" as being anything other than manufactured in order to intimidate you into giving them money.
Inform them that extortion is a crime, and you will be reporting their crime to the police - and actually do report their crime.
Making up laws and threatening you with them is not legal, particularly when these threats are accompanied by demands for payment.
Nothing will come of it, no one will be charged, but if you do end up in court you will be able to say you tried to protect yourself from the unlawful activities of their lawyers.
You can demand that the letters you've sent are read as part of your defense, hopefully convincing the court their belief in magic IP addresses, and the promise from a lawyer that you did it, should no longer be regarded as evidence of wrong doing.