reCAPTCHA is free, Google libraries based plugin that replaces default HostBill captcha image verification with Google reCAPTCHA.
To activate calendar plugin, visit Settings → Modules → Plugins → Inactive section of your HostBill, find and Activate "reCAPTCHA" plugin.
Proceed to https://www.google.com/recaptcha/admin#createsite to obtain Site Key and Secret Key required for module configuration