Mtn stole money out of my account

I requested cancellation of my Mtn contract and to switch to pre-paid on 1st June (ref ***1) and again 2nd June (ref ***) due to lack of response from Mtn. After endless battles, they finally cancelled my contract after 20th June and my phone was switched to pre-paid. The delay was through no fault of my own, whatsoever. I've just noticed that they've subtracted the full contract amount from my account at the beginning of July, then again at the beginning of August. This after I requested cancelling my contract 1st June! They'd assured me there was no waiting period for cancellation if I switched to pre-paid. They're literally *****ing money out of my account that I don't owe. I think it's about R450 that they've taken so far. I demand that the debit order be cancelled immediately and to be refunded in full! What utter incompetence, I just can't believe this company!