Users can mute the topic, that’s all I can think of at the moment.
You could run a Ruby/Rails command at the Rails command line to mute the topic for all existing users by updating their user preferences, but new users who sign up after you run that command will not have the topic muted.
Do note that only people with that topic in a “watched” state will get emailed for new replies, and even then, only if their web browser has not pinged the website in the last 10 minutes. So you could target this a little more surgically to update all user preferences from “watched” or “tracked” to “normal” on this particular topic. Perhaps @techapj could advise on the exact Rails command?