Code

...

    <ndiag:DiagramView runat="server"
		ID="diagramView"
		ClientSideMode="ImageMap"
		NodeModifyingScript="onNodeModifying"
		EnableItemsDOM="true"
        Width="800px" Height="500px" >
    </ndiag:DiagramView>

    <ndiag:InteractivityExtender runat="server" TargetControlID="diagramView" />

    </form>
</body>

<script type="text/javascript">
	function onNodeModifying(sender, args)
	{
		//alert("mod");
		var nodeRight = args.get_bounds().right();
		var nodeBottom = args.get_bounds().bottom();
		var viewDiv = sender.get_element();
		if (nodeRight > view.scrollWidth || nodeBottom > view.scrollHeight)
			args.set_cancel(true);
	}