Contract cancellation

I called and emailed a request to change banking details and migrate Sim cards to prepaid. This was in June, they said Sim will be migrated by 7 July. On my July pay day they debited the full amount, I sent an email again asking why, they said it was the last debit order, but this month they took the full amount again. So they ****, I don't understand why they are still debiting that amount, they should only debit for active contracts.