How to kill all active and inactive oracle 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# || ''' immediate';
  END LOOP;
END;

This should work - I just changed your script to add the immediate keyword. As the previous answers pointed out, the kill session only marks the sessions for killing; it does not do so immediately but later when convenient.

From your question, it seemed you are expecting to see the result immediately. So immediate keyword is used to force this.

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.