How to stop access using "/ as sysdba"

A friend of mine wanted to secure his client's database access by restricting the DBAs' connecting to the database as 'SYS' user by using "/ as sysdba".

In the file sqlnet.ora, located in $ORACLE_HOME/network/admin folder, add the following line:

SQLNET.AUTHENTICATION_SERVICES=(NONE)

Try logging in again by using "/ as sysdba":

No doubt this will prevent the access of "/ as sysdba" when connected as the 'oracle' user (oracle owner account) but the DBA can easily comment the configuration parameter in SQLNET.ORA - if he has the required permisssion on the file.
To avoid this, change the ownership of the sqlnet.ora file to 'root' or any other functional OS user, and provide a read permission to dba/oinstall group.

chown root:oinstall sqlnet.ora
chmod 640 sqlnet.ora