Heres where I got lost the first 20 times I was here. Scroll down until you see the "Media" category. Click on "View All Media". Once there, scroll through your media until you see the PLAYSTATION NOW SUBSCRIPTION, and click on that bad boy. On the next page, click the option that says PLAYSTATION NOW SUBSCRIPTION. It'll show you your billing dates here. Below all that is a box that says "Cancel Automatic Renewal". Now you have successfully unsubscribed.
Again, these steps were found using my cell phone. It may be a little different on a real web browser, but hopefully this will walk you through it
Easiest is on your computer log in to the PS website go to "Account Setting" then to "Media" and scroll to the date when you started the subscription and end the prepaid subscription from there.
I got the $9.99 for next month to test it out and let my kid play around with it as well. I made sure to turn off a unto renewal long before the month was over. But some how it was turned back on and I got charged the $20.
Did you turn off auto renew as part of my instructions on the first page, as if you turned off auto funding instead, it will still renew.