Block updates from salesforce sync

We have a certain custom field in Salesforce that identifies a Contact's status as a type of Prospect or Customer (User). Currently, this field is in the spreadsheet template we use to import a list of Leads into Marketo so that the person inputting these Leads doesn't have to go into SFDC and update fields she can update via this spreadsheet. For those that are new Leads, we do want to be able to import a particular value (Prospect C) for all of them. However, if a Lead already exists as a Contact in SFDC and is identified as a User, we do not want this field updated via the sync to Prospect C. I tried to build this into a smart campaign but it didn't work. I saw other discussions on Block updates from salesforce sync but I don't want this field blocked entirely, only conditionally.

So, my question is, does this feature block only actual updates (new values) to a field or all updates (no value to a value)?

Thanks!