FormCraft has a built-in set of methods to prevent spam. If you are still receiving unwanted form responses you can try these steps:

Require Access Code

You can lock your form behind an access code by going to Settings -> General -> Require Access Code.

form-protect-access-code-password

Disable Multiple Responses

This would prevent users from submitting the form twice. This can be enabled under Settings -> General -> Disable Multiple Responses.

Disable multiple responses entries

Captcha Field

Last resort is to add a captcha field, via Add Field -> Others -> Captcha.

Add a captcha field