how to set empty value for user's attributes such as subscriber with CSV import

I am trying to make an empty value for some users related attribute such as subscriber when we use CSV import from Eclipse client.

I know we can set "Unassigned" as a value for Owner or other attributes but my customer want to make it empty value rather than "Unassigned".

It is understandable that we can not make an empty value for owner or other attributes but when we create a new workitem, the subsciber's filed is empty.

It seems to be the import tool on Eclipse cached value and place old value if the value is empty on CSV files.
Is there any way to make subscriber empty rather than having "Unassigned"

Thank you!