Kill Session in RAC Database

You may get ORA-01940 even after you make sure that no user is connected by checking v$session.

If you are operating on RAC database, you probably should check gv$session and kill session on different instance. What Oracle will do is check sessions on the current instance you are connecting. So you will find nothing active there.

After you find the user connecting on another insntance through gv$session, you can kill that session by connecting to that instance if you are in versions before 11.
For Oracle 11, you can kill session in any instance by using this command:

SQL> ALTER SYSTEM KILL SESSION 'sid, serial#, @instance_id';