If you don't care about saving the open spreadsheets, this snippet will brutally close down all open instances of Excel.

#!perl -w
use strict;
use Win32::OLE;

my $ex = Win32::OLE->GetActiveObject('Excel.Application') or die "No a
+ctive objects around\n";
$ex->{DisplayAlerts} = 0;
$ex->Quit;
[download]

Another way to do it, at least on WinXP would be the system command.

taskkill /f /im EXCEL.EXE
[download]