A menu separator line is just a menu item. The following is from our response to your earlier post:
Formativ does not provide a means to delete GroupWise menu items. I will add your request to the Formativ Enhancements database.
However this is possible using the GroupWise Custom Third Party Object (C3PO) API. You can find out more about developing a C3PO in the Novell Developer Kit at Novell DeveloperNet. I believe the API function you require is CustomizeContextMenu.