Vodacom Overcharges after Payment already made

I've never had such bad service from Vodacom as what I've gotten the last 3 times I've called them. My account was in arrears which I settled on Friday. Vodacom then proceeded to debit my account 3 times this morning, none of these amounts being the amount that was actually owed for March. I only keep as much as I need to keep in my account so FNB then charged me for 2 transactions being rejected. This puts me out of pocket! When I spoke to the Santosh (the leader on duty this morning) he told me that its my fault for having my account in arrears. What is the point of making arrangements to pay if Vodacom will still debit. Why was I not told that I would be debited anyway even if I paid?? How can I pay the money to Vodacom and still have it available for them to debit from my account (even after making an arrangement to pay). I expect that the additional transactions are reversed and the FNB charged paid to me as well. The outstanding amount alone would have been covered if that is all that Vodacom actually debited. This is unacceptable! It may seem like a small amount to Vodacom but it is not for me, and is also just wrong to do nothing about it from Vodacom's side.