Only forms that are have never been published or are closed can be deleted. Once a form is deleted all submission data for it will also be deleted, so make sure you have exported your submissions before deleting a form.
If you want to delete a form, make sure you first close it for submissions. You can achieve this by clicking the “lock” icon on the “Forms” page or by pressing the “Close” button on the “Form Details” page of the form you want to delete.
Forms that have the state “unpublished” or “closed” have the option to be deleted on the same location as published forms have their “Close” action.