How to disable DraggingTool copy?

If the user holds down the Control key (Option key on Mac), the DraggingTool will make a copy of the parts being dragged.
How to disable this feature?

Set DraggingTool | GoJS API to false:

$(go.Diagram, . . .,
  { . . .,
     "draggingTool.isCopyEnabled": false
  })