Delete
There are two separate delete actions on a form's overview page: deleting individual submissions, and deleting the whole form.
Delete submissions
Select one or more submissions with their row checkboxes, then click Delete in the toolbar that appears. Confirm by clicking Submit -- this can't be undone.
Delete the form
Deleting the whole form (and every submission in it) lives inside Modify's Advanced Options section, not the main toolbar -- same pattern as Module Overview's Delete. Type DELETE (all caps) to confirm.
Warning:
Deleting a form removes every submission in it along with the form itself. If you only want to remove some submissions, use Delete submissions above instead.