Debit order problem

My son had a year contract with virgin active for a year. The contract ended on January 2015. A sms from virgin active stated that he is now a monthly client. February this year the debit order still went off his bank account. As he is a student he do not has the funds in his bank account by the first of every month. he went to the Stellenbosch branch to speak to the manager to stop the debit order. she told my son that virgin active can not stop the debit order nor can his bank details been removed from his profile. my son went to his bank and they told him virgin active has to stop his debit order as they can not. So the problem now is that my son pay his monthly fee every month cash by virgin active in Stellenbosch. By the first of every month the debit order still went off his bank account and there is unsufficient funds so the bank charge him a fee of R125.00. This happends about every month from February this year. This month it happened again by the first the debit order went off and bank charged him fee of R125.00 for unsufficient funds. He paid by cash at virgin active and now at the 15th of May there was a debit order set again that did not go off! What must I do?