mag@mutation_mutation > exec kill_allsession('MAG');
Alter System kill session(9,39974);
Alter System kill session(15,2111);
Alter System kill session(16,8);

PL/SQL procedure successfully completed.

mag@mutation_mutation > get kill
  1   create or replace procedure kill_allsession (p_username in varchar2)
  2   as
  3   cursor c1 is select sid,serial# from v$session where username=p_username;
  4   begin
  5   for mag in c1 loop
  6     exit when c1%notfound;
  7     DBMS_OUTPUT.PUT_LINE ('Alter System kill session('||mag.sid||','||mag.serial#||');');
  8  end loop;
  9* end;