In include\staff\tickets.inc.php simply remove the click close button.
onClick=' return confirm("Are you sure you want to close selected tickets?");'>
And in include\staff\viewticket.inc.php remove that part of the action form.
<? if($thisuser->canCloseTickets()){
//if you can close a ticket...reopening it is given.
if($ticket->isOpen()){?>
<option value="close" <?=$info=='close'?'selected':''?> >Close Ticket</option>
<? }else{?>
<option value="reopen" <?=$info=='reopen'?'selected':''?> >Reopen Ticket</option>
<? }
}?>
Then the only places left to close the ticket already require a message.