Some forms will have multiple sources of data. For example, Marketo's Munchkin API can overwrite data in fields. If you'd like to keep the initial data, Marketo allows blocking of updates to non-empty fields. The block action can also be narrowed by source.
Here are instructions for blocking updates to an existing, non-empty field:
1.1. Go to Admin and click Field Management
2.1. Find the field, select it, then under Field Actions, click Block Field Updates
3.1. Select the Input Sources you want to block and click Apply
This was taken from the Marketo documentation and further information can be found there.
Note: This method blocks updates to existing, non-empty fields. Empty fields will continue to be updated on first change.