The simplest thing is to set either Diagram.AllowInsert or Diagram.AllowCopy to false.
The more specific thing, to change the behavior of only the DraggingTool, is to override DraggingTool.MayCopy (and if WPF Windows-drag-and-drop is enabled, DraggingTool.MayCopyInternal and/or MayCopyExternal) to return false if the control key is held down.