Oracle - User to not see data from other users
This is the default behaviour in Oracle. Users do not have access to other user's objects unless explicitly granted.

The DBA role includes the "GRANT ANY OBJECT" privilege, allowing the user to see all database objects: 
https://docs.oracle.com/cd/B28359_01/server.111/b28310/dba005.htm
https://docs.oracle.com/cd/A97630_01/server.920/a96521/privs.htm

You should revoke the DBA role from every app user and stop granting it to new users.