Xing AG persistent demand money failure cancel account threatening lawyer letter Hamburg, Germany Internet

I was registered for Xing (a smaller competitor of LinkedIn) by a third party  several years ago.  I have never obtained any benefit from this registration and was alarmed to discover that I was paying for "premium features" with a monthly fee being taken from me.  I cancelled the payment and everything went quiet then I moved house and stupidly accessed my account again and updated my address.  This triggered a further demand for money which I ignored and sent an email within the "prescribed" timescale making it clear that I wished to cancel any subscription.  I have since received a number of demands for money and today I received a letter from KSP RECHTSANWALTE - a Hamburg lawyer firm who have trebled the amount demanded by adding their fees and further items - "Default interest, Prejudicial reminder costs and post and telecommunications expenses".

This is cleary a scam and I would advise anyone considering registering to avoid this company at all costs.  If anyone has succeeded in cancelling demands such as these please let me know.  The letter from the lawyer says "Once the total amount has been completely settled, the premium features can be reactiviated" - not what I want atall!

Any advice gratefully received as I fear that if I pay this demand it will only result in further demands for money.