REG in Ottawa

HELP! Some 2.5 years ago when Carleton Security (Ottawa) was selling its monitoring accounts to a Calgary company, I decided to cancel service, so Carleton staff removed the equipment. So, there has been no service for going on 3 years. In spite of that, ADT which appears to have acquired the accounts continues to insist on payment. We have many times tried to explain that I have not had the equipment so there is no service. ADT recently sent what they consider arrears to a collection agency which is pressing for payment and affecting my credit rating. ADT has been impossible to reason with, insisting that they have provided service years after it was disconnected. I'm 89 years old and do not merit this pig-headed predatory approach. I live in the same building as other family members and am still independent. How can I get ADT off my back and my credit rating restored? Mrs. RM, Ottawa