Google provides a free service called reCAPTCHA, Completely Automated Public Turing test to tell Computers and Humans Apart. reCAPTCHA helps protect your website against spam, abuse and can identify the difference between computers and humans. You can use this feature to protect your Signup Form from being abused by Spammers.
Before getting started with reCaptcha, please note that as of September 2018, many Google services were blocked in Mainland China. Due to these restrictions, reCAPTCHA may not work correctly if your visitors are in the China region. You can learn more about this by Clicking Here.
The reCAPTCHA feature can be located in the Small Embed, Embedded Form, and Popup signup forms. Please follow the steps below to turn on reCAPTCHA.
Select the type of form you would like to create and click Next.
Complete Step 1 Details and click Save & Next.
On Step 2 Design, click on Global Settings.
To enable reCAPTCHA, toggle the slider from gray to blue.
reCAPTCHA should look like this once it is enabled:
IMPORTANT
Disabling reCaptcha in a Popup signup form removes the Page Settings option. This is because, when a user clicks on the submit option in the signup form, they will be directed to another page to complete the reCaptcha steps. In the end, they only see the Thank you message for single opt-in or the double opt-in message if you have confirmation required.
reCaptcha option is per signup form if you enable it in one signup form, it does not enable in other signup forms.
Signup forms with ReCaptcha may not display correctly when viewed within the Facebook app. We advise that ReCaptcha not be used for signup forms if you plan to drive users to your webpages via Facebook campaigns or Facebook pages.
Signup Forms 
IMPORTANT 
SUGGESTION 
