How Can I Delete an Object From a Subdiagram Only?

You can delete an object from a subdiagram by selecting "Diagram | Add/Remove Objects".

In the check list that then appears, you uncheck the objects you want to delete from the subdiagram. Then click OK. The objects and related relationships will be removed from the subdiagram. Please note that the object is not deleted from the project. You'll still see it in the Main Diagram.

The Main Diagram will always hold all objects. Use "Edit | Delete" to delete an object from the project.