If the user clicks on any pickable GraphObject with the primary/left mouse button, the Diagram must raise the “ObjectSingleClicked” DiagramEvent. GoJS Events -- Northwoods Software
Whether you choose to listen to that event, or whether you choose to respond to that event, is up to you to decide.
So, if you don’t want to do anything, don’t do anything. Or remove that listener.
I think you understand the difference between clicking and selecting, but in case someone else reads this, please read GoJS Events -- Northwoods Software