Install and enable the following modules:
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
- recaptcha Choose version 2.x