Adt worst service

I cancelled my contract over 3 years ago. Then I started to receive messages that I will be blacklisted if I don't pay my account. I contacted the debt management and they assured me it was a mistake and sorted. Now I have started receiving the messages again!!! What is going on? I tried calling them this morning, but nobody picks up! I am so fed up with this. Ref 151736 for what it's worth!