You can cancel your free plan at any time for any reason without talking to anybody at Boombox.
Just access your account settings by clicking your avatar in the top right corner. Select "Account Settings" on the profile tab, and press "Delete Account"
Here's what you can expect:
1) You will not be able to log back in to your account.
2) Files and projects that have not been shared will be deleted.
3) Shared files, tasks, and comments will still exist, but without reference to your name or ID.
4) You will not receive any notifications related to past activities.