Being charged for a Product that was returned

Vodacom delivered a wireless modem to me end Oct '14. I tested it at home but couldn't get any signal. I phoned them and told them I don't want it anymore. They collected it on the 1st of November. Since then I've been billed every month for a product I don't have. I phoned them each month to fix the account. On the Feb '15 statement they gave me credit for the previous months but on the same statement billed me R99.00 again for the product. On March '15 statement they billed me again R99.00. Last month I sent an email to Vodacom and this morning again. Applicable Cell phone number ***