One tool that makes spam detection and prevention extremely easy is the Drupal 7 Mollom module. This module uses the Mollom service to give your Drupal website capabilities to add captchas to forms, or even analyse text for content that looks like spam.

To install and configure the Mollom protection settings for a form on your website:

1. Download the latest version of "Mollom" and install in your site
2. Go to Mollom.com and create an account.
3. Once you have logged in to Mollom.com, click "Manage sites" in the upper right menu.
4. Select "Add subscription" to create a new key pair for your website
5. Go back to your Drupal site and navigate /admin/config/content/mollom/settings
6. Enter the key pair that you created in 4.

   

7. For add mollom captcha to your form, Go here /admin/config/content/mollom and click on add form
8. Select your form and click on next

   

9. Configure Protection mode and other options and click on save

Once the module is active, you can log back in to Mollom.com in future to track how much spam has been prevented on your site.