Install and enable the following modules:

  • Webform

  • Options element

  • CAPTCHA Enable also Image CAPTCHA

  • admin->configuration->people->CAPTCHA check Add CAPTCHA administration links to forms

Now, when you create a webform, and save it, you will see (as admin): Show CAPTCHA: no challenge enabled.

  • click this link
  • click Place a CAPTCHA here for untrusted users.
  • choose the type of captcha

Probably you will want to disable the captcha on the login page:

  • admin->configuration->people->CAPTCHA->select No challenge for user login

More captcha's