If you're creating a form only accessible by members of your organization, chances are you're not worried about spammers. On the other hand, if your form is publicly available, and can be accessed anonymously, your form can easily become the target of spammers, hoping someone will read the content they submitted, or that this content will be published online, giving more weight to their site, through spamdexing.
Luckily, you can easily prevent spammers from exploiting your forms by adding a CAPTCHA. If you're writing your forms "by hand", in XForms, you can use the reCAPTCHA component. The component used the free reCAPTCHA service, provided by Google, and will show a challenge to your users filling the form, such as:
If you're creating your form with Form Builder, just enable CAPTCHAs for your form by setting the captcha-related properties, and Form Runner will take care of the rest for you, making sure that users entered a correct response before they can save or submit data.