Advance DataGird Header column Drag drop

Question

Hi.. I want to prevent drag drop in some column header .

Thanks

My code

private function _headerShift(evt:IndexChangedEvent):void{
                var dg:AdvancedDataGrid= AdvancedDataGrid(evt.currentTarget);
                var column:AdvancedDataGridColumn = dg.columns[evt.newIndex];
                var dropTarget:AdvancedDataGrid = AdvancedDataGrid(evt.currentTarget);
                if(evt.oldIndex > 3){

                    evt.stopImmediatePropagation();
                    evt.preventDefault();
                    dropTarget.validateNow();

                }
            }