Refunds changed to Debit Orders

I closed my 3 contracts with Cell C last month(August), and I paid cash. I received a paid up up letter, but I was surprised when I received a debit order of R1,402 on the 26 Sept 2015, and again this morning(30 Sept 2015) I was debited a huge amount of R6,241. I was really shocked, when I asked why? I'm being told that it was supposed to be my refund. I still don't understand how it was debited. This is very inconveniencing because it has put my account on a negative, and had to go to the bank to reverse it by paying R50 per debit (R100 for both). I would like Cell C to explain to why my account was debited after I had closed my account, and if this was meant to be a refund then why debited? I'm very irritated by this!!!