As ntozier wroted, if you mean new ticket form as Agent, then you need edit file \include\staff\ticket-open.inc.php.
Look for
if ($thisstaff->getRole()->hasPerm(TicketModel::PERM_REPLY)) { ?>
and change this condition to be wrong. For example
if (1+1==3 && $thisstaff->getRole()->hasPerm(TicketModel::PERM_REPLY)) { ?>