Hopefully you have kept a note of your previous calls to Wanadoo.
Make sure you have the exact details of the incorrect amount charged and what you consider to be the correct amount. You need details of package, cost, date payment due and amount actually charged. Sorry if I am repeating what you already know but just in case.
When you get through to Wanadoo explain that you have tried to get the refund on previous 3 occasions without a result. Request that you are transferred to the accounts department. You will then need to give full explanation and hopefully the matter will be sorted.
At each stage I suggest you note down the time, details of which call centre you are speaking to and of course the name and extension of the person answering your call.
I had a similar experience and I even e-mailed customer service. I received a reply saying that the matter is sensitive and can only be dealt with by phone. I did receive the refund due to me.