Billed for ADT account transfered 2 months ago

I moved houses 3 months ago. My ADT contract was transferred to the new tenant. ADT confirmed this in writing. Yet for the past 2 months I have received an invoice for the amount of R2378.25! Last month I called ADT to follow up on this invoice. I was told my account balance was in fact R0.00. Yet I have now received another invoice for the same amount this month. I called ADT again, they could not in fact now confirm whether my contract had been transferred and what the balance on the account was. As if ADT's shocking service delivery was not enough during my contract, I now have to continue to be billed for it! The worst part of this situation is that if I do in fact owe this amount ADT will easily get me blacklisted, for an amount which I would be happy to settled if only ADT to confirm what is actually owed!