It can be rather annoying if you are notified in your activity stream that user xyz has updated excel file abc for the 124th time... If you click on "Stop Following" it can happen that you are told "You are not currently following any items for this story". "You are seeing this update because you are related to the story in some other way. For example, the story might be related to a person in your network or it might be a response to content that you posted."
Please give the users the control over the activity stream back. A users should be able to unfollow these items.