Blocking SQL Server db_datareader, db_datawriter, and db_owner Permissions

n a database I currently support, the application makes use of db_datareader and db_datawriter to give permissions to the tables. In a different database, it's even worse as the application uses db_owner. We're adding new tables to both databases and we don't want the application to have access to these tables. How can I deal with the use of these three roles?