unauthorised contract

I had a contract with ismart it ended 1st September 2014 it was fully paid. I phoned in and stoped the contract the number is now pay as you go. After almost 2 year they send me a message that my acount is in arrears January 2016. I phoned in they said that my contract was paid in full September 2014. A month l received an other sms saying my account is arrears again. I phoned them an said that its not my contract and i did not give any one authorisation to open a new contract. Now that keep on harassing me about the contract that i don't have I'm not receiving any air time or any thing els. I spoke to the acount superviser on 17 march 2016 he said that they will cancel the contract after they tried to deduct an amount of R150 out of my bank account this was the firts time they tried to deduct the money the contract have been active a lot longer than that. I will not pay any thing i haven't given any permission or autherisation to open the contract. Ismart has opend a contract for someone on my old account that i had with them.