Found it:
/include/class.ticket.php Line 702 - 714
//set status to open on a closed ticket.
function reopen($isanswered=0) {
$sql='UPDATE '.TICKET_TABLE.' SET updated=NOW(), reopened=NOW() '
.' ,status='.db_input('open')
.' ,isanswered='.db_input($isanswered)
.' WHERE ticket_id='.db_input($this->getId());
//TODO: log reopen event here
$this->logEvent('reopened', 'closed');
return (db_query($sql) && db_affected_rows());
}
Can just modify the above to not reopen.
If you really wanted to, you could probably add a checkbox of you own to the actual form that would set whether to reopen the ticket or not. The code already exists on the /scp side.