To un-protect object from accidental deletion, you need to un-check proper box in “Object” tab in OU properties. In order to see this tab, you need to configure the ADUC by going to the View option on the toolbar, and then check “Advanced Features”.
This will add some new containers and tabs in OU properties. In “Object” tab in OU properties you will see check box “Protect object from accidental deletion” – un-check the box and you should be able to delete OU.