The easiest way to cancel your monthly payments is to login to your members page here and choose 'Payments History' then click on the link 'Cancel Subscription'. You can also access the page directly here.
(If you can't remember your login details, please see "I can't remember by login details - Username and Password")
This will take you directly to a page in PayPal with a cancellation link for subscriptions to Zapp English. Click that link, and your subscription payment will be cancelled.
You will still have access to everything you have paid for up to that point, but no new downloads will appear in your members area.