Created a logon trigger,but can't prevent a user from logining to the database

1.logined to the database with sysdba and created following trigger:

SQL> conn / as sysdba

Connected.

SQL> create or replace trigger limit_logon

AFTER LOGON on database

declare

dbrole varchar2(16);

loginuser varchar2(30);

begin

loginuser:=sys_context('userenv','current_user');

select database_role into dbrole from v$database;

if dbrole='PRIMARY' and loginuser='A' then

raise_application_error(-20999, 'Cann''t logon this database.');

end if;

end;

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Trigger created.

2.create a user named as a and do login test

SQL> create user a identified by oracle ;

User created.

SQL> grant connect,resource to a;

Grant succeeded.

SQL> conn a/oracle;

Connected

It showed that the trigger can't prevent user a from logining.