Same situation happened to me. However, this is what I did.
I called the customer service number and verbally went around in circles with the first rep about the ridiculous charge and the "15 day" trial period. Then I got sick of dealing with her so I demanded to speak with her supervisor.
After going around in the same circles with her, she confirmed that my "membership" had been canceled and that I would not be charged again, to which I replied that "I will be talking with my bank to ensure no charges go through from the Natures Best company". At which point, she asked if I intended to dispute the previous charge, which I confirmed. Immediately, she said that she would provide me with a full refund of the charges already made.
She verbally gave me a confirmation number on the refund, membership cancellation, as well as confirmation numbers for two separate SPAM membership cancellations. I also requested those confirmation numbers by email as well, which I have received. This just happened, so we'll see if the refund is ever credited, but at least for the time being, you all can attempt to do the same.