Easy way to prevent a user from unchecking a checkbox in a datagrid

Hi i have a checkbox datagridcolumn inside a datagrid, how can i prevent user from unchecking the checkbox located in first row without creating a whole itemrenderer etc..

//someArray is bindable

<mx:DataGrid  id="somegrid" dataProvider="{someArray}" >
  <mx:columns>
    ....etc...
        <mx:DataGridColumn dataField="isChecked" rendererIsEditor="true" editorDataField="selected" >
               <mx:itemRenderer>
	<fx:Component>
	      <mx:CheckBox   click="outerDocument.displayCheckBox_clickHandler()" />  
                   </fx:Component>
               </mx:itemRenderer>
        </mx:DataGridColumn>	
</mx:columns>
</mx:DataGrid>

public function displayCheckBox_clickHandler():void
{
        if(somegrid.selectedIndex==0)
       {
              // set the filterArray[0].isChecked = true
              // basically check the checkbox again, but this does NOT work, the filterArray[0].isChecked is set to false again.
             // i think due to the way the events are ran.  
             //This is my question, how do you prevent a user from unchecking the checkbox in first row of the datagrid
            //OR  to auto recheck it after they uncheck it.        
      }
}

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