Step 1. Create an account with reCAPTCHA

Go ahead and create an account, download the PHP API (and copy recaptchalib.php in the _includes/ directory, and sign up for an API key.

Step 2. Add reCAPTCHA when posting an ad

config.php
Define 2 constants, one for public and one for the private API key:

define('CAPTCHA_PUBLIC_KEY', '12345_YOUR_PUBLIC_KEY');
define('CAPTCHA_PRIVATE_KEY', '12345_YOUR_PRIVATE_KEY');

Also add the following line (find the other require_once lines), which includes the recaptcha library:

require_once '_includes/recaptchalib.php';

_templates/publish-write.tpl
Right above the submit button’s fieldset, insert the following code:

<h2 class="publish_section">Anti-spam</h2>
<fieldset>
  {literal}
    <script type="text/javascript">
      var RecaptchaOptions = {
        theme : 'white',
        tabindex : 9
      };
    </script>
  {/literal}
  {$the_captcha} <span class="validation-error">{if $errors.captcha}
  <img src="{$BASE_URL}img/icon-delete.png" alt="" />{/if}</span>
</fieldset>

page-write.php
On line 2, we generate the html code that displays the captcha in the template:

$smarty->assign('the_captcha', recaptcha_get_html(CAPTCHA_PUBLIC_KEY));

Next, we perform a simple validation when posting the ad. Find the following line:

if ($_POST['action'] && $_POST['action'] == 'publish')

A couple of lines below it, after you initialize the $errors array, add this code:

$resp = recaptcha_check_answer(CAPTCHA_PRIVATE_KEY,
$_SERVER["REMOTE_ADDR"], $_POST["recaptcha_challenge_field"],
$_POST["recaptcha_response_field"]);

if (!$resp->is_valid)
{
  $errors['captcha'] = 'Incorrect code';
}

Then, we need to add the captcha validation in another section of page-write.php, the one that handles the ad edit. Find the following line and add the same code as above:

else if ($_POST['action'] && $_POST['action'] == 'edit'

That should be it .
Enjoy!