I've two pages inside a WordPress installation Body having a MailChimp register form, after which another page which shows a thanks message in which the user is distributed after finishing the MailChimp register form.

At this time, the thanks page is simply a regular WordPress page. While you will find no links into it within the navigation, you'll be able to go to the page directly if you possess the URL.

However, I have to get this to page to ensure that it can't be utilized unless of course the register form was posted, i have to make certain the page can't be rejuvenated. Wouldso would Time passes about doing that?

Put this on the top of the page.

The shape action on page one should be the 'thank you' page.

<from method="POST" action="page2.php">
Your form input.
<input type="submit" name="submit" />
</form>

Page 2:

if(!isset($_POST['submit'])){
    echo('You hacker!');
}else{
    // Rest of the page
?>
    <html> etc.
    thank you bla bla....
<?php
}

Make use of the referrer in the $_SERVER variable.

Inside your thank-you page, put something similar to this

if ($_SERVER['HTTP_REFERER'] == 'mailchimp_host') {
  echo 'Thank - you ';
  ....
}
else {
  echo "Not authorized to view this page";
}

Substituite mailchimp_host using the real hostname from the mailchimp server that's doing the subscription from the user. Help you log files!