I am trying the below script to kill all active and inactive oracle sessions for user at once but it doesn't work. The script executes successfully but does not kill sessions for user.
BEGIN
FOR r IN (select sid,serial# from v$session where username = 'USER')
LOOP
EXECUTE IMMEDIATE 'alter system kill session ''' || r.sid
|| ',' || r.serial# || '''';
END LOOP;
END;
The KILL SESSION command doesn't actually kill the session. It merely asks the session to kill itself. In some situations, like waiting for a reply from a remote database or rolling back transactions, the session will not kill itself immediately and will wait for the current operation to complete. In these cases the session will have a status of "marked for kill". It will then be killed as soon as possible.
Check the status to confirm:
SELECT sid, serial#, status, username FROM v$session;
You could also use IMMEDIATE clause:
ALTER SYSTEM KILL SESSION 'sid,serial#' IMMEDIATE;
The IMMEDIATE clause does not affect the work performed by the command, but it returns control back to the current session immediately, rather than waiting for confirmation of the kill. Have a look at Killing Oracle Sessions.
Update If you want to kill all the sessions, you could just prepare a small script.
SELECT 'ALTER SYSTEM KILL SESSION '''||sid||','||serial#||''' IMMEDIATE;' FROM v$session;
Spool the above to a .sql file and execute it, or, copy paste the output and run it.