Are you looking just to cancel your subscription and downgrade to the free plan? See http://help.freebusy.io/subscriptions-and-billing/how-do-i-cancel-my-subscription
To close your account:
Go to Subscriptions.
Scroll to the Close Account section at the bottom of the page.
Click Close Account.
Closing your account does not affect your actual calendar data. Your calendar data resides with your calendar provider, not with FreeBusy.
Important: If you have an active subscription to a paid plan, you will also see an option to Close Account and Stop Recurring Billing. Choosing this option will also stop recurring billing for your subscriptions. If you don't choose this option you will continue to be billed when your subscription is due for renewal. To know when your subscription is due for renewal:
Go to Subscriptions.
Click on your subscription under Manage Subscription section.
Go to Licenses tab.