The Admin is the only user who has permission to delete a Workflow.This user can delete the workflow only if the status is in Draft
In order to view workflows list Log into Admin.
Click on "Manage" tab.
Click on "Workflows" item in the sub-menu.
Click on the icon shown in the below screen shot.
Click on Delete Workflow option from the drop down menu.
NOTE: Published flows cannot be deleted. Change the workflow status to draft to see Delete button.