Disable WordPress email based on subject or any other variable

disable_wp_mail_by_subject.php

<?php
add_filter('phpmailer_init', 'stopCartNotifications');
function stopCartNotifications( $phpmailer ) {
    // Let's extract object variables so we can use them.
    $variabile = get_object_vars($phpmailer);
    // I' blocking based on subject so, i'm selecting Subject from the object below in phpmailer_object_data.php
    $subiect_email = $variabile['Subject'];
    // I'm searching for Contact from the blog... (Calculated Form Fields)
    if ( !empty($subiect_email) && $subiect_email == 'Contact from the blog...')
        // Well, the subject matches, let's clear the email.
        $phpmailer->ClearAllRecipients();
}

phpmailer_object_data.php

<?php
// This is what $phpmailer from above contains. You can basically 
PHPMailer Object
(
    [Version] => 5.2.22
    [Priority] => 
    [CharSet] => utf-8
    [ContentType] => text/plain
    [Encoding] => 8bit
    [ErrorInfo] => Invalid address:  (addAnAddress Reply-To): 
    [From] => [email protected]
    [FromName] => [email protected]
    [Sender] => 
    [ReturnPath] => 
    [Subject] => Contact from the blog...
    [Body] => 
    [AltBody] => 
    [Ical] => 
    [MIMEBody:protected] => 
    [MIMEHeader:protected] => 
    [mailHeader:protected] => 
    [WordWrap] => 0
    [Mailer] => mail
    [Sendmail] => /usr/sbin/sendmail
    [UseSendmailOptions] => 1
    [PluginDir] => 
    [ConfirmReadingTo] => 
    [Hostname] => 
    [MessageID] => 
    [MessageDate] => 
    [Host] => localhost
    [Port] => 25
    [Helo] => 
    [SMTPSecure] => 
    [SMTPAutoTLS] => 1
    [SMTPAuth] => 
    [SMTPOptions] => Array
        (
        )

    [Username] => 
    [Password] => 
    [AuthType] => 
    [Realm] => 
    [Workstation] => 
    [Timeout] => 300
    [SMTPDebug] => 0
    [Debugoutput] => echo
    [SMTPKeepAlive] => 
    [SingleTo] => 
    [SingleToArray] => Array
        (
        )

    [do_verp] => 
    [AllowEmpty] => 
    [LE] => 

    [DKIM_selector] => 
    [DKIM_identity] => 
    [DKIM_passphrase] => 
    [DKIM_domain] => 
    [DKIM_private] => 
    [DKIM_private_string] => 
    [action_function] => 
    [XMailer] => 
    [smtp:protected] => 
    [to:protected] => Array
        (
            [0] => Array
                (
                    [0] => [email protected]
                    [1] => 
                )

        )

    [cc:protected] => Array
        (
        )

    [bcc:protected] => Array
        (
        )

    [ReplyTo:protected] => Array
        (
        )

    [all_recipients:protected] => Array
        (
            [[email protected]] => 1
        )

    [RecipientsQueue:protected] => Array
        (
        )

    [ReplyToQueue:protected] => Array
        (
        )

    [attachment:protected] => Array
        (
        )

    [CustomHeader:protected] => Array
        (
            [0] => Array
                (
                    [0] => X-Mailer
                    [1] =>  PHP/7.0.16
                )

        )

    [lastMessageID:protected] => 
    [message_type:protected] => 
    [boundary:protected] => Array
        (
        )

    [language:protected] => Array
        (
            [authenticate] => SMTP Error: Could not authenticate.
            [connect_host] => SMTP Error: Could not connect to SMTP host.
            [data_not_accepted] => SMTP Error: data not accepted.
            [empty_message] => Message body empty
            [encoding] => Unknown encoding: 
            [execute] => Could not execute: 
            [file_access] => Could not access file: 
            [file_open] => File Error: Could not open file: 
            [from_failed] => The following From address failed: 
            [instantiate] => Could not instantiate mail function.
            [invalid_address] => Invalid address: 
            [mailer_not_supported] =>  mailer is not supported.
            [provide_address] => You must provide at least one recipient email address.
            [recipients_failed] => SMTP Error: The following recipients failed: 
            [signing] => Signing Error: 
            [smtp_connect_failed] => SMTP connect() failed.
            [smtp_error] => SMTP server error: 
            [variable_set] => Cannot set or reset variable: 
            [extension_missing] => Extension missing: 
        )

    [error_count:protected] => 1
    [sign_cert_file:protected] => 
    [sign_key_file:protected] => 
    [sign_extracerts_file:protected] => 
    [sign_key_pass:protected] => 
    [exceptions:protected] => 1
    [uniqueid:protected] => 
)