Roku's typically link these days by sending you an email with a link that you click on.
If you are being charged via Roku, then you would cancel it in your Roku account (at https://www.roku.com/) – note that this would be the Roku account that you had when the charge was made – I say that because it sounds like you are trying to create a new Roku account, but that won't stop your old Roku account from having active subscriptions. Hopefully you still remember how to get into the old account. (You can try the Forgot Password? Link at the bottom of the sign-in page.)
