Free accounts can NOT be deleted. “It should not be made too easy” (haha).
However, paid accounts can be deleted: “You can cancel your account at any time by clicking on the Cancel Account link in the payments section of the site navigation. All of your Content will be immediately deleted from the Service upon cancellation. This information can not be recovered once your account is cancelled. If you cancel the Service before the end of your current paid up month, your cancellation will take effect immediately and you will not be charged again.”
So here all of a sudden it’s easy to delete your account. One conclusion: customers get in easy and switch to a paid account, but cannot leave (unless they’ve paid).
Perhaps it helps to contact them using the form found here: https://www.backupify.com/contact