Change the email subject for a single form.

add_filter('frm_email_subject', 'change_subject', 10, 2);
function change_subject($subject, $atts){
  extract($atts);
  if($form->id == 45){ //Replace 45 with the ID of your form.
    $subject = 'Thank You'; //Replace Thank You with your email subject
  }
  return $subject;
}