I did this just this morning (before removing it just as quickly : )
Go into ./include/staff/viewticket.inc.php and find:
<? 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>
<?}
}?>
And either remove it or replace it with:
<? /* 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>
<?}
} */ ?>
(I.e., replace the code or comment it out)
HTH :-)