Robbing clients

Homechoice does not stick by any agreement.. Verbal or written.. A payment arrangement was made with them as I am on maternity leave. A minimum installment of R1000 is being paid towards the account every month but they are still debiting my account.. They have debited R3500 from me yet I paid as per agreement.. Every month they change the reference number so I cannot even put a stop order.. This is exhausting for me please someone assist