I want my money back

We signed a 24 months contract with ADT for an alarm system in our house. The 24 months period lapsed, I have made telephonic and written requests to cancel the contract but they keep debiting my account and ignore my emails.

When we signed the contract we were advised that the device was rent to buy device but now they tell us a different story. We were also told that they were responsible for ensuring the device was always in good working condition during the 24 month period. The device had so many problems and were were always told that we must pay a call out fee which we refused to pay. The device still has problems, keeping the contract is a financial burden. We also have upgraded security in a complex, private security is unnecessary. I want my refund from the period I requested cancelation and for them to stop debiting my account.