Get the bank to do a chargeback on it if you haven't authorized the charges. If its not on your account, VF cant tell you whose it is or refund it. If you did authorize it then vodafone will prove it to the bank but chances are your card details are just being used by someone who saw the card to top up their phone.
Sorry, I wasn't clear in my original post - that's the first thing I tried. To be clear, the payment is going towards my prepay account - I just didn't set it up, and I can't stop it. I spoke to ASB and they said that they couldn't do anything. They told me to contact Vodafone, and if that didn't work, I'd have to cancel my credit card.
Before anyone asks what the problem is if the money is giong onto my Vodafone account - the amount charged each month is greater than the amount i spend each month, so the balance on my Vodofone account will continue to increase as long as I keep it active. I'd rather top up my account manually when it needs it, which is what I have been doing for the last six or so years.