Sudhi,
As you are probably aware, if an operating system user is a member of the known Oracle DBA group (example: "ORA_DBA" on Windows or in *nix it is the group mentioned in the ".ascii" entries of your "$ORACLE_HOME/rdbms/lib/config.s" file), then they have
implicit privileges to "connect / as sysdba" without using a password.
Therefore, there are 1 or 2 methods to prevent a user from logging in successfully with "connect / as sysdba" without a password. Method 1: cause no operating-system users to have membership in the privileged DBA group, or, Method 2: (for *nix) Step A) edit your $ORACLE_HOME/rdbms/lib/config.s file to refer to a bogus, empty *nix group, then Step B) "relink all", to re-link all Oracle software components with the new, "empty-membered" Oracle DBA group.
You will need to ensure that you have a properly created Oracle password file (so
somebody can log in with a password and do sysdba activities).
Let us know if this answers your questions.
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Mufasa
(aka Dave of Sandy, Utah, USA @ 17:50 (13Jul04) UTC (aka "GMT" and "Zulu"), 10:50 (13Jul04) Mountain Time)