By default export to excel is enabled when navigator is enabled. The difference in using export to excel in the jqGridRender class, as opposed to export to excel in the jqGridClass, is that the names in header, width and initially hidden columns are taken from column model generated in the jqGridRender class. In that respect, you have more control on the export and how the Excel spreadsheet is formatted upon exporting.

To disable the export to excel in navigator you should set the excel property to false.

$grid->setNavOptions('navigator', array("excel"=>false));

Additionaly you can overwrite the export file name to excel using the exportfile variable:

$grid->exportfile ='myexcelfile';