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.
Disable Multiple Responses
This would prevent users from submitting the form twice. This can be enabled under Settings → General → Disable Multiple Responses.
Last resort is to add a captcha field, via Add Field → Others → Captcha.