What you ask is not possible by default. The simplest solution would be to close the topic after it has been created and not allow other answers from users (but only from the staff or specific groups) or close the category and set permissions for users to see, but not reply or open new topics. In this way you would have substantial control of the topics and replies sent via email.
Otherwise another solution could be to set the category to require moderation approval of all new replies.
