Blacklisted for account that has been payed

We stopped used ADT two years ago due to bad service and problems with accounts. We made the last payment in May 2014. We still receive sms's threatening us to pay and even received a phone call this morning that we will be blacklisted. This is for an amount that was paid! When we phone, they ask for proof of payment, that has been sent three times, and promises to phone back. Surprise, surprise! That never happens....