ORA-01940: cannot drop a user that is currently connected

SQL>drop user username cascade;
drop user username cascade

*
ERROR at line 1:
ORA-01940: cannot drop a user that is currently connected

SQL>select s.sid, s.serial#, s.status, p.spid from v$session s, v$process p where s.username = 'ISL_DF' and p.addr (+) = s.paddr;

SID           SERIAL# STATUS    SPID

----------   ----------    --------       ------------
159               29          INACTIVE 12349

SQL>alter system kill session '159,29';

System altered.
SQL>!kill -9 12349            - (kill the process in OS level)
SQL>drop user username cascade;
User dropped.