In the Attribute Presentation tab in Diagrams Options it is possible to define the prompt message for an element type. Then save the settings and open the diagram.

Drag and Drop the element into the diagram. A prompt with an OK and a Cancel button is displayed. This also shows the message which was defined in the attribute settings.

Click the Cancel button, and depending on the defined setting, the element that was dropped into the diagram will not be defined and is deleted.

To enable the prompt window, the prompt must be defined in the Attribute Presentation tab.

Click on the Cancel button. If the Cancel button option is enabled in Diagrams Options the element will not be defined in the database and the shape will be removed from the canvas.