Maybe someone from Jive can chime in, but I too do not see a way to turn off project updates in the same way you can with docs, bookmarks, blog posts, etc.
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Has this person chosen to "Receive e-mail notifications"? If so, this may be a reason they are receiving the notification. Also, did they receive a notification on a task they were assigned, or just on a piece of content in the project?
If they are following a particular individual, they will receive a notification when that person creates content in a Space or Group they have permissions to. However, I'm not sure if this notification trumps the other notifications they have an option to change.