Unauthorized and Unannounced Charge At End of Free Trial

I signed up for a free trial with GraphicStock.com to see if it would be worth my time (and money) to opt for a subscription. I can't complain about GraphicStock's portfolio in and of itself, but there was nothing that particularly matched my needs and so I decided I would not opt to continue using their services after the free trial expired. I was given no notification that my free trial was about to expire, nor any option to discontinue my account. Instead, I had $51 removed from my bank account without so much as a please and thank you! Let me stress the fact that when I signed up for the free trial, I was assured that there would be NO charges whatsoever, and that I would be given the option of cancelling if I decided not to continue with my account after the free trial expired. Instead, it was assumed (with NO correspondence on the matter) that I would be signing up for a paid account, and the money was withdrawn without my authorization. This is absolutely unacceptable. Even if it would have been worth my time to pay for an account moving forward, I will not be using GraphicStock.com ever again nor recommending them.