PL/SQL Procedure to Kill a session

Hi all, I was trying to create a stored procedure but i doesn't seem to work please help me in doing this, I'm trying to write a procedure to kill the session of a particular users and my code is


create or replace procedure testkill ( p_user varchar2)
IS

den varchar2(100);
v_count number(6) := length(regexp_replace(p_user, '[[:alpha:]]', '')) - 2;
query varchar2(20000);
BEGIN
for i in 1..v_count LOOP
den :=substr(p_user,
((instr (p_user, ',',1,i)+1)),
(((instr (p_user, ',',1,i+1))) - (((instr (p_user, ',',1,i)+1))))
);

query := 'select sid ||' ||''','''|| '|| serial# from v$session where username like ''' || '%' || den || '%''' ;

dbms_output.put_line( query);

execute IMMEDIATE('alter system kill session ' ||''''|| query || '''');

END LOOP;

END testkill;

This procedure throws error when executed. If I execute this query,

EXEC testkill (',GOPI1,HR,')
it throws error, i'm a beginer in writing procedures please provide me a solution or assist me in fixing this procedure.