Blocking a Form From Overwriting Stored Data

I am trying to figure out a way to stop a form from overwriting data that we already have stored for that user.

Specifically, if a user fills out a form and changes the value, "How Did You Hear About Us?", we want to block the form from filling in that data with new information in order to preserve the original answer to that question when their profile was created. The initial value was probably set by a form so having the field blocked entirely from form updates in Marketo is probably not the solution.

So, we need to be able to stop this field from updating only if the original data field is not empty, and only for this field. All other fields in the form should update normally.