SQL> ed
Wrote file afiedt.buf

  1  CREATE OR REPLACE TRIGGER
  2   t1_disable_one_row
  3  BEFORE UPDATE  ON t1
  4  FOR EACH ROW
  5  begin
  6    if :old.id=1 then
  7     raise_application_error(-20101,'Строку з с ид='||:old.id||' запрещено менять');
  8    end if;
  9* end;
SQL> /

Trigger created.

SQL> select * from t1;

        ID N
---------- ----------
         1 Мажит
         2 Stax

SQL> update t1 set n=n||' Ок!' where id=2;

1 row updated.

SQL> update t1 set n=n||' Ок!' where id=1;
update t1 set n=n||' Ок!' where id=1
       *
ERROR at line 1:
ORA-20101: Строку з с ид=1 запрещено менять
ORA-06512: at "STAX.T1_DISABLE_ONE_ROW", line 3
ORA-04088: error during execution of trigger 'STAX.T1_DISABLE_ONE_ROW'


SQL> update t1 set n=n||' Ок!';
update t1 set n=n||' Ок!'
       *
ERROR at line 1:
ORA-20101: Строку з с ид=1 запрещено менять
ORA-06512: at "STAX.T1_DISABLE_ONE_ROW", line 3
ORA-04088: error during execution of trigger 'STAX.T1_DISABLE_ONE_ROW'


SQL> select * from t1;

        ID N
---------- ----------
         1 Мажит
         2 Stax Ок!

SQL>

SQL> select * from t1;

        ID N
---------- ----------
         1 Мажит
         2 Stax

SQL> CREATE OR REPLACE TRIGGER
  2     t1_disable_one_row
  3  BEFORE UPDATE  ON t1
  4  FOR EACH ROW
  5  begin
  6      if :old.id=1 then
  7        if updating('ID') then
  8          :new.id:=:old.id;
  9        end if;
 10        if updating('N') then
 11          :new.n:=:old.n;
 12        end if;
 13        -- .... другие поля
 14      end if;
 15  end;
 16  /

Trigger created.

SQL> update t1 set n=n||' Ой!';

2 rows updated.

SQL> select * from t1;

        ID N
---------- ----------
         1 Мажит
         2 Stax Ой!

SQL>