This worked for me :
$workbook.Close($false)
$excel.Quit()
[System.GC]::Collect()
[System.GC]::WaitForPendingFinalizers()
[System.Runtime.Interopservices.Marshal]::ReleaseComObject($workSheet)
[System.Runtime.Interopservices.Marshal]::ReleaseComObject($excel)
Remove-Variable -Name excel