Spurious emails generated

My site has crashed, and keeps crashed, because Appointments Lite is sending out hundreds of “reminder” and “thank you” emails for non-existent appointments. The error message that I have been getting is ………….
“Resource Limit Is Reached The website is temporarily unable to service your request as it exceeded resource limit. Please try again later.”

Please see one of the emails that I have received, from Appointments Lite………..

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a temporary error. The following address(es) deferred:

healyjoe1@securesmtp.stream
Domain pro-holistic.co.uk has exceeded the max emails per hour (530/500 (106%)) allowed. Message will be reattempted later

——- This is a copy of the message, including all the headers. ——
Received: from uncannyt by unicron.3v0.net with local (Exim 4.88)
(envelope-from <uncannyt@unicron.3v0.net>)
id 1corDy-001iuy-7t
for healyjoe1@securesmtp.stream; Fri, 17 Mar 2017 12:43:20 +0000
To: healyjoe1@securesmtp.stream
Subject: Thank you for your enquiry
X-PHP-Script: http://www.pro-holistic.co.uk/index.php for 46.161.9.20
X-PHP-Originating-Script: 641:class-phpmailer.php
Date: Fri, 17 Mar 2017 12:43:14 +0000
From: Pro-Holistic <enquiries@pro-holistic.co.uk>
Reply-To: enquiries@pro-holistic.co.uk
Message-ID: <61d692bbfa09cea4d567cc6096cdc071@www.pro-holistic.co.uk>
X-Mailer: PHPMailer 5.2.22 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
X-OutGoing-Spam-Status: No, score=0.0

Thank you for your enquiry. We will reply as soon as possible.

I have, eventually, managed to access the C Panel and delete the Appointments Lite file. I am not waiting, with fingers crossed, in the hope that the site comes back to life.