Ticket abort send how to do??

Hi i try abort send tickets for staff. I'm try use code below, but $merge_fields['abortsend'] = true;
not work. Have you idea hot to fix.

<?php

function hook_pre_send_email_demo($vars) {

$email_template_name = $vars['messagename']; # Email template name being sent
$relid = $vars['relid']; # Related ID it's being sent for - client ID, invoice ID, etc...

$merge_fields = array();
$merge_fields['my_custom_var'] = "My Custom Var";
$merge_fields['my_custom_var2'] = "My Custom Var2";

$merge_fields['abortsend'] = true; # You can use this return to stop email sending

return $merge_fields;

}

add_hook("EmailPreSend",1,"hook_pre_send_email_dem o");

?>