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 = 'CHRS'
and p.addr (+) = s.paddr;  2    3    4

       SID    SERIAL# STATUS   SPID
---------- ---------- -------- ------------------------
       139      17563 INACTIVE 21630
       142      56779 ACTIVE   10920
       148      20995 INACTIVE 27150

SQL> alter system kill session '142,56779';

System altered.