No contract but money being deducted

I've been struggling for a very long time with mtn/express telecoms, with regards to an upgrade that I wanted. In the end, the address where I wanted the package delivered couldn't be verified, so I cancelled the upgrade, and my entire contract with mtn! Since then, 3 payments of R559.00 each have been deducted from my bank account, without my authority, after I terminated my contract, and after I cancelled the upgrade! How dare they deduct money from someone's bank account illegally! Is there anyone here who can advise me how to take this matter further (legally), because I haven't signed anywhere for anything. On behalf of mtn I have to say, their response is timely and efficient, and they always help. Express telecoms on the other hand, I struggle to get in touch with! If anyone here reading this can help me I would really appreciate it. Thank you in advance!!!!