Null field value if no value is given on form submit

We are trying to set up a process where if a person submits a form, and a non-required field is left empty, we infer that means the person no longer has that item.

Example: Bill told us 4 years ago he grows Rice. This year he renews his subscription to our magazine and leaves Rice blank (we cannot pre-fill per print audit rules). We want to have Marketo wipe out his previous answer and assume he no longer grows rice.

Can that be accomplished?

I thought perhaps have some sort of trigger campaign but that will overload our system and create a log jam in our Campaign Queue.

Ideas?