Change the DiagramView.Behavior property. Its default value LinkShapes lets you draw new ShapeNode and DiagramLink objects. You could set Behavior = DrawLinks to allow only drawing new links, e.g. if nodes will be created via drag and drop. Other values you might try are Modify to allow only selecting and moving items, or DoNothing if you need to show a read-only diagram.

I hope that helps,
Stoyan