Click on Add new and choose String Field.
Give the field a name and in the field help, explain what you want the user to do.Something like, "to prove you are human type on the word cardinal in all lower case into the box below."
Be sure and click the required box.
Click on Overrides and fill in the data under Custom Validator.
You can just cut and paste the following if you wanted to use cardinal as your test word:
python: test(value=='cardinal', False, 'Go away Robot!')
Don't for forget to hit Save.
You can change the cardinal and Go away Robot! to anything you want, just leave the ' marks.
If someone fills the field in correctly, the form is sent normally.
If someone (or thing) failed to follow instructions the form doesn't go and they get a reply explaining why.