How to delete positions from a position hierarchy ?

Hello,

We are on R12.1.3

We are using positions hierarchy for requisition approvals.

There are many old, obsolete positions available in the position hierarchy.

some of them does not exist in the hierarchy itself ( as the 'Exists In Hierarchy' check box is unchecked )

The delete button is greyed out too.

What is the easiest way to clean up and keep the ever changing position hierarchy accurate.

----Thanks,

Anais