Billed twice in one month and it seems like a downgrade is difficult

I have used Vumatel for over a year now without hassles. On the 24th of September I asked for a downgrade to pay R250 instead of the R400 a month and I was told to pay the R250 which i paid on the 25th. On the 1st of October R400 was again deducted on my account, when I sent them a message on their WhatsApp chat to ask for a refund I was told that the R400 cant be deducted since the service is active and they would have to refund the R250. I asked for a downgrade and the lady said that she has done the downgrade on her side and it will be effective by the 1st of November. It is now the 1st of November and AGAIN they have deducted R400 from my account. After sending confirmation that I have been downgraded. My R250 hasn't been refunded to date. I have spoken to over 9 consultants on their WhatsApp and I have sent more than 5 statements showing that I've paid these amounts and again when I enquired today the are still asking for the very same proof of payment I sent in October. This is very unprofessional and very frustrating. This is my last resort since I've spoken to so many consultants and nothing is being done!