How to kill session when system could not drop database user?

Please execute following commands as "sys user":

select inst_id, sid, serial# from gv$session where username = 'TEST_ALPER';
(inst_id -> instance id, sid -> service id, serial# -> serial number)

then use;

alter system kill session '44,61808,@1';
(44 -> inst_id, 61808 -> service id, @1 -> serial number)

finally drop user;

drop user TEST_ALPER cascade;