Who can I complain to about Groupon?

Last week somone hacked my groupon account and I received a confirmation email for a purchase I didn't make. I immediately called Groupon to let them know that I didn't place this order and was told the order would be cancelled, the fraud team would look into it and I would receive a refund after up to 5 working days.
After 5 working days I still hadn't had a refund so called up again. I was told that the order wasn't actually cancelled and that I shouldn't have been told I would get a refund as the fraud team had to contact me first. The advisor said that I had been sent an email from them and when I said I didn't receive anything he told me another email would be sent within 5 working days.
Surprise surprise today is day 5 and no email.
After the second call I phoned my bank to let them know about the fraudulent payment and was informed that the person actually tried two other larger purchases which were refused due to insufficient funds, groupon never mentioned this to me or sent me emails telling me payment was refused so I had no idea. The visa fraud team are looking into it but before they can force Groupon to refund me they need proof that groupon think the transaction was fraudulent. Any idea who I can complain about this to as I'm quite angry that the order wasn't cancelled straight away when I told them and also that they are dragging their feet over giving me a refund!