this is a feature not a bug - you have to select Import all + Delete old recipients in the import task to get what you need.
More info here
Sorry, I just realized that your problem is a bit different.
This functionality has been added in SR4.
Fields were not updated after Recipient Import
When importing recipients with columns populated and mapped, the import worked fine, but if you imported some recipients again with some columns cleared they were not updated and the related fields remained filled. This is because the import recipients functionality didn’t manage the cancellation of field values. Starting from this release NPrinting has the new flag "Consider empty values" in the import recipients task, that permits clearing of the values in the recipients when the source has empty values.