Sorry to see you go! To delete Handle, simply delete the app. Its possible that one notification may come up after you delete, but they will stop after that. You may notice that Handle created folders in your Gmail account - you can delete those manually, since Handle no longer touches them.
Handle stores none of your email and to-do data, so there's no trace left.
To delete Handle calendars, launch the Calendars app on your iPhone or iPad.
Tap on Calendars at the bottom > Edit > Select calendar to be deleted > tap delete calendar > tap on delete calendar again on the popup. On your computer, in your Gmail/Apps accounts, go to calendar, calendar settings, and delete the Handle Due Date calendar in each by clicking 'unsubscribe.'