For Prestashop 1.7 it's in another file.
controllers\admin\AdminCustomerThreadsController.php
change from:
$new_ct = (Configuration::get('PS_SAV_IMAP_CREATE_THREADS') && !$match_found && (strpos($subject, '[no_sync]') == false));
to:
$new_ct = (Configuration::get('PS_SAV_IMAP_CREATE_THREADS') && !$match_found && (strpos($subject) == false));