FormIt will use reCaptcha:
https://rtfm.modx.com/extras/revo/formit/formit.hooks/formit.hooks.recaptcha, but if you want to use Captcha, you would have to add the image yourself (with something like the code below) and write a custom postHook to test the value by comparing the user's input to the $_SESSION['veriword'] value. Return true if they match and false if not. Put the postHook ahead of any postHooks that do anything (e.g., email).
<img style="border: 1px solid #039;" src="assets/components/captcha/captcha.php?rand=rand()" alt="If you have trouble reading the code, click on the code itself to generate a new random code." width="148" height="60" align="top" />
If it's a simple contact form, SPForm will use Captcha if you tell it to.