In some cases, automatically clearing the field is a good option, but in other cases I would consider it "user abuse". If someone mistypes a credit card number, for example, they will want to go back and fix the typo rather than type in the whole thing again.
The way LiveCycle handles validations is to bark at the user as they exit the field, but allow the user to continue filling out the form. However, LiveCycle will prevent the user from submitting the form if there are validation errors. The user can still save and can still print. Often my clients want to prevent printing and saving unless the form is valid, but there's no simple way to do that. One option is to put a conspicuous marker on the form (DRAFT or WORK IN PROGRESS watermark on the master page, for example) that indicates that the form is incomplete. You can clear the watermark when the form is valid (i.e. xfa.form.execValidate() returns true).
Jared Langdon
http://www.jlangdon.ca