Hi Eric,
One possible solution to this could be in downloading the mdl_user table from the database (if you have PHD MyAdmin or Moodle Adminer available).
In this table, you'll find columns for lastmodified, lastaccess, lastlogin etc., all with a date/time stamp in Epoch style. This can be easily converted to something more understandable.
Then use this filtered table as a basis for your upload users CSV file. That way, you can delete or suspend users that haven't been online for a while.