avoid sys user, disable oracle log
There is no way to limit the abilities of the SYS user. It is a super user account, similar to root on a UNIX operating system.

If a user changes the logging method, this act will be logged. The last OS-level log will show the act of changing it. So your DBA will not be able to disable logging without leaving a record.

Note that the auditing features of Oracle are greatly improved in version 12c. If you can upgrade, you probably should.