Apaled by the service

I had a cell c number which i owed R11 on (airtime advance). Two years ago i ported the number and started receiving smses from cell c regarding the amount owing. I settled the amount, visited Cell C and was told to email the proof of payment. I have done this for the past 2 months now and i a still receiving the smses to say i owe them money. Every time i email them, someone asks me to send the proof of payment again and again and again. i do! and still nothing is being done.