If you have Data Bridge you can add captcha to the form
to add the acptcha form element go to Webassist -> Forms -> Insert Captcha
your form should already have server validation applied to it, go to the Server Behaviors panel, and double click the Server Validation server behavior to edit it and add the Like Entry validation for the validation element.
Select the Like Entry Validation type, for the server variable, click the lightning bolt and select the captcha form element from the bindings panel, the click the plus button to add it to the validation list, for the comparison, click the lightning bolt icon and select the captcha session variable.
use the validation show if behavor to show the error message on validation failure.