If you wish to clear, or delete, data for Subscribers during an import, you must use the SmartSubscribe import.
Any data file in your import file that has been mapped to a Subscriber or Custom data field is updated during import. Whatever data is in the file will be saved to the Subscriber. There are two options to delete any data already stored for the Subscriber; set the field data in the import file to blank (empty field) or use the string 'NULL'.
The 'NULL' string and an empty string in the import field are both treated as the command to delete the content in the field, or store blank data.
Be certain to only map fields that you wish to update during the import, because changing or clearing data cannot be undone.