How to Forbid SYS and SYSTEM to Access Product Schema Data with Oracle Standard Edition?

My production database is Oracle Standard Edition. 

Oracle Database Vault is not available for Oracle Standard Edition.

How can we protect the whole production schema to be accessed by the SYS/SYSTEM (or any with DBA role) users, including SELECT, INSERT, UPDATE and DELETE?  

We developed an application system for our customer with Oracle Standard Edition database. The customer manages the SYS/SYSTEM password for maintenance and backup purpose. Our application stores sensitive data under one PROD schema, and our application system use the schema owner as application user to login the database to manage the data through application. 

The problem is the customer using the SYS/SYSTEM user to login the database to update the data bypass the application system, which always causes the data error. They also use the  SYS/SYSTEM user to select large volume of data that cause the application performance issue.