ADT Claim I owe money after i cancelled 3ys ago!

I am totally disgusted in the way ADT have dealt with this! Three years ago I cancelled my contract as I moved premises due to the fact that they could not protect my family and I . I moved and cancelled the account including writing a letter of disgust in the lack of service delivery to the head office I received a call back from head office stating they would deal with it and they are so sorry! Now three years later I just got slapped with the debt collection letter for over R8000!!!! *******... on there it states I owe R2000 fro a decoder & R3500 Systems cost which is also ******* as I moved into the property and it already had the alarm in it and this came at my own expense. No where in the last three years has anyone from ADT Head office nor collections/ accounts department tried to call me to tell me about this. My post box still works and has not changed and neither of my phone numbers either so they have no excuse!!! ADT were in breach of their contract right from the beginning as they could never arrive on time or within the 5 min stipulated on the contract. THIS IS A TOTAL DISGRACE TO ADT AND I WOULD ADVISE TO NEVER DEAL WITH THEM EVER!!!!!!