Prezi Charged Me for One Year Subscription - I Want Refund

I initially signed up for a Prezi trial, and they charged me $55.20 without my authorization, which is completely unacceptable. I canceled on the second day of my free trial because it was not what I was looking for, yet I was charged for a full one-year subscription despite only using the service for about two hours before canceling. I had no further activity on their software, and I immediately requested a refund, but I still have not received my money back. I demand an urgent resolution, as this is not a small amount, and I will not accept unauthorized transactions on my account. If my money is not refunded immediately, I will take legal action. Prezi, return my money now!