Just go to Account >> Plan & Usage and then select either the "free" plan if you would like keep access to your data, but not be charged, or "permanently delete your account," if you would like all your data and billing info wiped from our servers.
Your card will no longer be charged (see below). The change will be effective immediately, but does not work retroactively. For example, if you realize you were just charged for a month, and cancel a few days later, your credit card will never be charged again, but you will not get a refund for that last month. You will always have access to your data without paying a dime, but you do have to remove any ethnio code from your servers, or you'll hear from us, rather quickly. And you won't be able to pay incentives using Ethnio without an active paid account. If you want to completely delete your account, like how we wish Facebook would allow us to delete an account, then simply drop us an email saying you would like your account completely deleted. That would be permanent deletion - wiped from all backups and never to be seen again.