Disable creation of new nodes

I found that if I press “CTRL” and drag an original node, a new node will be created just like the original one, but I don’t want to allow any new node, where do I disable this feature?

You are talking about the behavior of the DraggingTool to allow users to make copies of selected Parts. You can disable that specific behavior by setting DraggingTool.isCopyEnabled to false:

$(go.Diagram, . . .,
    "draggingTool.isCopyEnabled": false

For more discussion about the various possibilities, please read GoJS User Permissions -- Northwoods Software