Deducting money from my account without my consent

I took out a cellphone contract in May. When I got home there was no cellphone reception. Reported problem within the 7 days of contract. 2 months later a technician confirmed this. Telkom mobi phoned me suggesting I cancel the contract and they will not deduct penalty fees. They how ever take every month 12-10 thousand rand from my account without my concent. This is theft. I have phoned them they apologize and just keep on doing this.