Google offer a free service - reCAPTCHA - that helps separate bots from humans. You've probably seen this in action many times:
All hosted forms on EmailOctopus include reCAPTCHA protection by default, helping protect your forms from spam submissions. To enable this protection on your embedded form, simply select the option in the form builder before generating the code:
We use the latest version of Google reCAPTCHA, which doesn't require the user to check a checkbox. In fact it often won't appear at all - it aims to be as unobtrusive as possible.
You can verify that the protection is enabled by checking the bottom right of your browser - you should see a reCAPTCHA logo: