You could try and visit the lawyers office, as this is very important for you. You can try and call the lawyers office, is it´s far. However, you might need to show up in person anyway to cancel the will. I do not think they will do that by e-mail nor phone.
As you say, in your country, like in mine, the last will rules. However, here it might be different and it will be much easier for Thai´s to contest your wishes. It might also be that the will held by the lawyer reign supreme or the Amphur. You should really do more than just send a mail.