By removing all your Ad Accounts, from the Ad Account page on AdEspresso(check here on how to do that), we remove from the application all the data concerning your Ad Account and any connection with your Facebook account.
The only thing that last on our DB is your email, that is the username used to access the platform (which can be changed any moment).
We store some data for backup reasons, but all are periodically whipped out (and not accessible from outside).
You can also revoke permissions for the AdEspresso App at any moment, by going to the Business integrations page in your Facebook settings. There you'll see a list of all the app you're using and should also see AdEspresso, that you can easily remove.
-- This operation do NOT remove any data from Facebook! --
All your campaigns and Custom Audiences will still be available on Ads manager and Power Editor.