Blocking updates to a field allows you to write to the field once and then retain the original value for the lifetime of the field. This can be useful for a field like Person Source.
Admin Permissions Required
Go to Admin and click Field Management.
Find the field, select it, then under Field Actions, click Block Field Updates.
You can block updates to Program Member Custom Fields as well.
Select the Input Sources you want to block and click Apply.
When performing a list import, the status of a field being blocked in the Import Preview will only show if the field is automatically recognized by Marketo based on the name of the field matching exactly (or if aliases are established). If the field is manually chosen from the Marketo Field drop-down, the blocked status will not show in the Import Preview, but update-blocking to that field will still be implemented.