To cancel your Unlimited Class Plan, go to your Account Settings (located in the dropdown menu when you click on your avatar, upper right corner). Click on the Cancel button in the Subscriptions information box located on the right side of the page (or bottom of the page if you are on a mobile device).
Click on the Cancel link, and follow the rest of the instructions.
A cancellation confirmation email will then be sent to your email address.
You can also find the Subscription information box on the Prices page, and proceed with your cancellation from there.