ADT keeps taking money from my account

We cancelled our ADT contract in May 2015. <br> There was an outstanding amount which I paid in July.<br> Then at the end of September, they debited my account. I phoned an fought with them about it, and had the money refunded, but that took 2 weeks.<br> And now again, at the end of October, they have debited my account AGAIN!!!<br> This is theft, taking money out of my account when our contract is cancelled.<br> No one can give me a straight answer and I keep getting pushed from pillar to post on who is responsible.<br> Can anyone tell me how to get ADT to stop billing me?