I've been looking for a way to stop Vuforia engine from tracking, and googleing didn't help at all.
I wanted to stop tracking when I found a tracker, and then resume trackng after an event (like a button click).
This is what I used to accomplish what you need. (:
TrackerManager.Instance.GetTracker<ImageTracker>().Stop();
TrackerManager.Instance.GetTracker<ImageTracker>().Start();