myButtonBar_btb.addEventListener(IndexChangeEvent.CHANGE, onIndexChange);

private function onIndexChange(e:IndexChangeEvent):void
{
	// The button we want to check here is the third one in the menu so the e.newIndex = 2 for this button.
	if( e.newIndex == 2 ){
		// noActivation_btn is the button that links to en external file/webpage.
		var noActivation_btn:ButtonBarButton = myButtonBar_btb.dataGroup.getElementAt(myButtonBar_btb.selectedIndex) as ButtonBarButton;
		noActivation_btn.selected = false;
 
		// activateThis_btn is the current active button in the menu			
		var activateThis_btn:ButtonBarButton = myButtonBar_btb.dataGroup.getElementAt(e.oldIndex) as ButtonBarButton;
		activateThis_btn.selected = true;
	}
}

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" 
			   minWidth="955" minHeight="600"
			   creationComplete="init(event)">
	<fx:Declarations>
		<!-- Place non-visual elements (e.g., services, value objects) here -->
	</fx:Declarations>
	<fx:Script>
		<![CDATA[
			import spark.components.ButtonBarButton;
			import spark.events.IndexChangeEvent;
 
			private function init(e:Event):void
			{
				myButtonBar_btb.addEventListener(IndexChangeEvent.CHANGE, onIndexChange);	
			}
 
 
			private function onIndexChange(e:IndexChangeEvent):void
			{
				if( e.newIndex == 2 ){
					var noActivation_btn:ButtonBarButton = myButtonBar_btb.dataGroup.getElementAt(myButtonBar_btb.selectedIndex) as ButtonBarButton;
					noActivation_btn.selected = false;
 
					var activateThis_btn:ButtonBarButton = myButtonBar_btb.dataGroup.getElementAt(e.oldIndex) as ButtonBarButton;
					activateThis_btn.selected = true;
				}
			}
		]]>
	</fx:Script>
	<s:ButtonBar id="myButtonBar_btb" horizontalCenter="0" top="20">
		<s:dataProvider>
			<s:ArrayCollection source="['Normal button', 'Normal button', 'You cannot activate me']" />
		</s:dataProvider>
	</s:ButtonBar>
</s:Application>