I phoned them wanting to find out about the debit they did,it took over two months to get hold of them

I phoned them wanting to find out about the debit they did,it took over two months to get hold of them.They deducted the amount we were not agreed to,so I cancelled the contract but they still continue to try debit into my account Any day of the month, when they don't get the money my bank charges 150 rand.It affect my credit score.My debit only happens on the 15 of each month.