(I suposed it's a SQL sentence).
Yes, with this you can change all users to 0
UPDATE `phpbb_users` SET `user_notify` = '0';
Remember to make a backup bevor.
And change phpbb_ to you prafix
I think this only helps for new topics. ("Notify me when a reply is posted" is unchecked)
For the old ones you have to change it also in DB for every post.
I will check this