Lynda,
If you go to the admin section of Marketo and you can edit field update settings. Click on the "
Field Management" section and then you should get a list of available fields on the right side.
You can drag them into the center area. Next, click on the "
Field Actions" dropdown and click "
Block Field Updates" (red circle next to it). From there, you can select the types of field updates you wish to block for that specific field.
You'll have to do this process for all the fields you want to block updates from on list imports...Also this will apply to all future list imports, not just a one-time import.
This article:
http://community.marketo.com/MarketoArticle?id=kA050000000KypCCAS gives you a good rundown of the process as well.