vodacom wants payment for amount never mentioned

Payment arrangement was made with Vodacom, the account was paid in full int Augsut 2015, they kept on deducting weekly payments, after emails and fone cals that extend over 3 month period they refunded the amount of R463. This was 30 October, the same day they deducted another R80 again! We can not stop the payments, its a external stop order. And Vodacom must stop it! In 2 emails to me they stated the order was removed. But it was'nt as payments keep on being deducted. On 16 November I received a fonecall we owe R2700 for handsets! Never was this amount mentiond when payment arangements was made, and never have I received any statement of this amount, not even while the corosponding with the over deductions! How can my account be in credit and they refund me, but I must pay antoher R2700, that was never mentioned before! I've got all my emails stating the amount will be refunded and the external order has been stopped! When asked how is this posible the explanation was, it will keep on deducting in credits until the R2700 is paid how is this possible!And to my knowledge if you sighn a selfone contract you sign for one ammount, and this ammount we paid of + ammount in rears.