create or replace trigger trg_lock_scott
after logon 
on schema 
declare
stmnt varchar2(100):='alter user '||ora_login_user||' account lock'; 
Begin 
if ora_login_user='SCOTT' then 
execute immediate stmnt;
end if ;
End ;

create or replace trigger trg_lock_scott
after logon 
on database 
declare
stmnt varchar2(100):='alter user '||ora_login_user||' account lock'; 
Begin 
if ora_login_user='SCOTT' then 
execute immediate stmnt;
end if ;
End ;

30511, 00000, "invalid DDL operation in system triggers" 
// *Cause:  An attempt was made to perform an invalid DDL operation
//          in a system trigger. Most DDL operations currently are not 
//          supported in system triggers. The only currently supported DDL 
//          operations are table operations and ALTER?COMPILE operations. 
// *Action: Remove invalid DDL operations in system triggers.