Here is the issue:
Person A creates a post on the home page activity stream
Person B comments on that post
The problem is now Person B gets notifications for all new comments on that stream. I see no way to turn that off. I went into Email and Notification preferences and turned each one off one by one, and the only one that stopped the emails were to turn off system notifications.
Also, it does not appear that there is an "unfollow" setting for each individual post?
The site I shared is a dev server, so you can create as many test comments as you like. You can also create a dummy account to test if you need.
Thanks!