When adding a captcha control to a PageBuilder Form widget the captcha does not prevent the form from being submitted when the captcha is wrong or blank.
This is a bug that has been identified and fixed within Ektron. In order to get this fix you will need to upgrade to Ektron 9.2 SP2 CU2 or newer. You can find the latest Service Packs and Cumulative Updates on the Ektron Product Downloads page. If you are not able to upgrade you will have to use smart forms or create your own custom form to use.