A PHPExcel object can't easily be destroyed because the hierarchy of spreadsheet->worksheets->cells contains cyclic references in that the cell also references the worksheet, and the worksheet references the spreadsheet, so special methods are needed to clear it from memory (as described in section 4.3 of the developer documentation,
This should free up a more memory from the PHPExcel object for you
Clearing a Workbook from memory).
$objPHPExcel->disconnectWorksheets();
unset($objPHPExcel);