When trying to delete an object from AD, you get this error:
The directory service can perform the requested operation only on a leaf object
This is because the object you are deleting contains another object, making the object a (sort of) container. This can even be true of computer and user account objects! To confirm this, in AD Users and Computers (dsa.msc), click View -> Users, Contacts, Groups, and Computers as containers. You will then be able to expand the object to see if it has anything inside it.