Since sending an email to client to inform him/her about response for the ticket is a mandatory feature, there is no option to disable or enable it from Admin Panel. In addition, the current version of osticket system is not an account-based-system, therefore, each client does not have his/her preferences/setting for this.
If you still want to disable this feature, open \include\class.ticket.php and FIND:
Misc:($this->getEmail(),$subj,$body,$from,$fromName,$headers);
REPLACE WITH:
// Misc:($this->getEmail(),$subj,$body,$from,$fromName,$headers);
Alternative way that you can do is add a "Send email to client" checkbox on the form in \include\staff\tickets.inc.php (with checked setting recommended), then add new code before that code above, give an If - Else conditional; If checked then send email to client, Else: do nothing.
Best regards,
Masino Sinaga