Unauthorised debit order

I cancelled my upgrade contract after noticing that I cannot afford to pay for the R3700 they requested and promised me that no debit order will run because contract is cancel and surpassingly they went on and debit that R3700 Is a debit check can't be reversed they can't even tell me how and when will I get my refund I feel that they are not professional their contract deposit on pre-owned phone is too much and not necessary