Hi Simmy - If you want to prevent a form from being submitted based on a value, you'd need to work with a developer to add add validation code to the form, there's not a out of the box feature for that.
See number 6. on the developers page for this: http://developers.marketo.com/documentation/websites/forms-2-0/
For custom error message you can change the default for some fields Change the Language of Form Validation/Error Messages and Submit Button - Marketo Docs - Product Docs
Or if it's custom you would also need to use code. See number 9. on same Forms 2.0 developers page.