Context
When a form has multiple sources of data, the data from one source may overwrite another. For example, Marketo's Munchkin API can overwrite data. If you'd like to keep the initial data, Marketo allows you to block updates to non-empty fields. The block action can also be narrowed by source.
Steps
Follow instructions from Marketo's article.
Outcome
Marketo blocks updates to existing, non-empty fields. Empty fields continue to be updated on first change.