The enhancements in Spring '17 release will allow you to enable spam filtering with reCAPTCHA from Google.
As per normal Web-to-Lead setup, navigate to Setup | Customize | Leads | Web-to-Lead, click "Create Web-to-Lead Form". Tick option for "Enable spam filtering (recommended)", then you need to create reCAPTCHA API Key Pair.
How to create reCAPTCHA API Key Pair?
1. Make sure you have Google account.
2. Navigate to Google reCAPTCHA page, login to Google account, and click Get reCAPTCHA button.
3. Enter label and domain name, make sure this is domain that you will use to host Web-to-Lead HTML.
4. Copy the Site Key and Secret Key.
Now, back to Salesforce setup page to enter reCAPTCHA API Key Pair created. Click lookup icon in reCAPTCHA API Key Pair, then click New button, enter API Key Pair Nickname, Secret Key and Site Key created earlier. Click Save button to store the keys for Web-to-Lead.
As per normal, continue with selecting fields to show in the web form, and the return URL.
- Getting Started reCAPTCHA
- Google reCAPTCHA
- Generate Leads from Your Website
- Web-to-Lead: reCAPTCHA Web Form Validation