Cell C is *****ing money from our bank account!!

My husband gave Cell C proper notice to cancel his contract after two years. They took months to acknowledge the cancellation and every month a "final" payment had to be made. After the "final" debit order gone through, they still deduct the same amount every month. My husband went into the bank to cancel the debit order but was told that they cant cancel it as Cell C only changes the reference number on the next month's debit order and that it will go through. We contacted Cell C to which they reply that they can't see that the money is deducted by them and that we have to bring in statements. My husband again reversed the debit order in July on the 27th. On the 31st the same amount was debited by Cell C again. Cell C is *****ing money from us as we did cancel our contract from them and they keep on debiting the same amount. This results in banking costs for us as the money is debited, reversed, debited and so forth. This is absolutely unacceptable and I am considering legal action. Theft is theft, even if done by debit order.