Drawing out a region only stops detection of alerts if you specify that ALL objects in the region are uninteresting. If you leave at least one object as interesting, detections will still occur, and you'll be able to see those detections as "uninteresting alerts" by enabling uninteresting alerts in the alert filter.
So, a bit hacky, but you could pick a pretty uncommon object (e.g. maybe bear depending on where you live) and leave it as interesting in the area that you marked off as uninteresting.