CANCELLED SERVICE IN 2014 STILL RECEIVING INVOICES

I cancelled our service at our business in March 2014 but I kept on receiving a invoice for an outstanding amount of R510.94 November 2015 I decided to pay the R510 although I was not sure what this invoice exactly are for I just wanted to get it done with. But now I receive a new invoice again monthly for R510.94 and I dont even have the alarm system in anymore as it was removed by ADT when I cancelled in 2014. I have eversince sent numerous emails but no response from ADT Please assist asap<br> <br> Regards,