Oracle Forms

How can I programmatically enforce the ‘Update Only If NULL’ = Yes item property in Forms 6?

If I have this property set against an item the form will allow the item to be updated, provided the original value was null prior to commit. As soon as the record is committed the field can no longer be updated if it is not null. However, I can still update the field programmatically within the form, bypassing the Update Only If NULL validation. I need to be able to enforce the Update Only If NULL property no matter how the field is updated.