create trigger tu_YourTable on YourTable for update as
begin
declare
@numrows int,
@numnull int,
@errno int,
@errmsg varchar(255)

select @numrows = @@rowcount
if @numrows = 0
return

/* Non modifiable column "MiddleName" cannot be modified */
if update(YourColumn)
if exists (select 1
from inserted i, deleted d
where i.YourColumn != d.YourColumn)
begin
select @errno = 30001,
@errmsg = 'Non modifiable column "YourColumn" cannot be modified.'
goto error
end

return

/* Errors handling */
error:
raiserror @errno @errmsg
rollback transaction
end

Оппа, Power Designer генерит триггер с ошибкой, а я раньше этого не замечал
Код
if update(YourColumn)
if exists (select 1
from inserted i, deleted d
where i.YourColumn != d.YourColumn)
надо заменить на
if update(YourColumn)
if exists (select 1
from inserted i, deleted d
where i.ID = d.ID and i.YourColumn != d.YourColumn)