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.
In Marketo, go to Admin and click Field Management.
Find the field, select it, then, under Field Actions, click Block Field Updates.
3. Select Sources
Select the Input Sources you want to block and click Apply
Marketo blocks updates to existing, non-empty fields. Empty fields continue to be updated on first change.
This was taken from the Marketo documentation and further information can be found there.