Sorry to reply to an old thread - however, I got requested to do the same thing today and could not find a thread with the right answer.
To close this one out if anyone needs to do the same thing - it is indeed in class.ticket.php.
Look for function postMessage and then look for the following lines:
if(!$this->isOpen()) {
$this->reopen();
//If enabled..auto-assign the ticket to last respondent...if they still have access to the Dept.
if($cfg->autoAssignReopenedTickets() && ($lastrep=$this->getLastRespondent())) {
//3 months elapsed time limit on auto-assign. Must be available and have access to Dept.
if($lastrep->isAvailable() && $lastrep->canAccessDept($this->getDeptId())
&& (time()-strtotime($this->getLastResponseDate()))<=90*24*3600) {
$this->setStaffId($lastrep->getId()); //Direct Re-assign!!!!????
}
//TODO: Worry about availability...may be lastlogin also? send an alert??
}
}
Remove them all and this will do the trick