form builder uses Spry validation for providing immediate feedback to the user on filling in he form, but also uses server validation to ensure that it is filled in.
spry validation or any other client validation type should not be the only validation you rely on to make sure the form is filled in.
to use server validation in your form, go to WebAssist -> Forms -> Validation -> Server Validation.
in the server validation behavior, select the validation type to use, then click the lightning bolt icon next to Server Variable and select the form element to validate, then click the plus button to add it to the validation list. repeat this for each form element you wish to validate.