You can disable the zooming of the Diagram by setting the Zoomrate property to 0:
<telerik:RadDiagramID="RadDiagram1"runat="server"ZoomRate="0">// Omitted for clearance</telerik:RadDiagram>
For more information about disabling/adjusting the zooming in the Diagram, I suggest taking a look at the following article - Disable the Viewport Zooming in RadDiagram.