Useless and greedy ADT

Cancled my contract in September to the first of December. As it happens to others. It is now claimed that I owe an old payment which is not true. The last step from them is now a rude layer who threatened me with legal action and blacklisting. The only one who should be out of SERVICE is a useless security company which does not deserve to be in security services. Even the police is faster when it comes to a call out.