Harassed for an ADT account that was cancelled in 2017

Every 4 months I receive a phone call about an ADT account that was closed in 2017. Each time I send proof of the cancellation and ask for the calls to be pulled to prove that there is no monies owed and each time it is ignored. Then 4 months later I get another call and I start the whole process again. No one seems to want to help or correct this issue and this has been going on for years now. Please can someone senior, who doesn't work in a call centre, contact me to resolve this urgently.