Charged for services not rendered

Due to poor service, Contract with ADT was cancelled. I followed proper procedure as per contract. I placed notice on 28 April 2015. As I pay for my service in advance as per contract, my debit order went through on 25 April 2015 for the month on May 2015. Thereafter my account was debited again on 25 May 2015 and 25 June 2015. On 15 June 2015 I received a refund for payment made on 25 May 2015. However, despite my numerous emails to ADT, to-date I have not received a response or refund for 25 June 2015. According to an email received from the person i dealt with, she stated that the reason why they cannot refund the monies is because my payment are made in arrears. However this is contradicting what is written in the contract and I also make my payments in advance.I do have the correspondences and names if required