I had to look around to find out what a followed thread is, I must admit it's a feature I've not seen before. (You click on your name and then select "Follow Lists", for anyone else unaware). You can delete entries but I can't see anything to stop threads from being automatically added when you reply to them.
I did spot the second-to-bottom option in preferences:
"Send me an email when a new post to a forum, thread, or by a user within my Follow Lists is created"
and in my case, it is set to "No". If you use that feature, I can see you'd want to have more control over what is added to the lists.
