Disabling the Tools->Cancel menu item of Purchase Orders window

 Hi
I've setup a restricted purchasing responsibility by defining a custom menu:

Seq Prompt                            Function     
10   Requisition                       Requisitions   
20   Requisition Summary        Requisition Summary   
30   Purchase Order                Purchase Orders
40   Purchase Order Summary Purchase Order Summary
60   Profile                              Profile User Values
70   Notifications                     Universal In-Box
80    Reports                          Requests: Submit
85  Funds Inquiry                    Funds Available Inquiry
90                                         PO Copy Document
100                                       PO Summary: Open Document
110                                       PO Summary: Create New PO   
120                                       PO View Invoice Summary

Users of this responsibility can't access the Tool->Control menu of Purchase Order Summary windows
so that they can't cancel a PO.
Unfortunately the menu item Tools->Cancel on the Purchase Order window  is still active.
How can I disable it?