For the Flat File source, to set an account as enabled or disabled based on the values inside a specific column, you can add the IIQDisabled (string) attribute in the source account schema. For more information on editing the account schema, refer to Managing Source Account Schemas.
This is the companion discussion topic for the documentation at https://documentation.sailpoint.com/connectors/flat_file/help/integrating_flat_file/account_status.html