Crunchyroll engineered a scam. If you payed using paypal, you have to cancel through paypal. It's ridiculous.

From the main page, Crunchyroll.com, go to "Profile" on the top right of the page, and then click "Settings" from the dropdown menu. On the left-hand side of the page, you'll see the option "Premium Membership Status," and once clicking on that, you should be able to cancel. I found it a bit confusing, because it seems to say you can have access to your premium account until the end of your billing cycle, but it didn't work like that for me -- so keep that in mind.