If you want to delete an organization, you should close that organization first.
The owner of the organization is the only person who can delete it. If you are not the owner, you can leave the organization.
Once you've closed the organization, open the Closed Projects page.
Find the organization, click on the "Settings" and select the "Delete Organization".
For deleting, you must write the name of the organization in the box, then the delete button will be activated and you can delete the organization.