I have a friend that cut up her finger so bad she had to go to the emergency room for stitches.
The way I learned to keep it shut was from a favorite teacher at our LQS, The Quilting Bee. I have taken many classes from her and also private lessons. I left my cutter open on the table one day, and she found it. All in fun, she came over and playfully, lightly slapped my hand. She does it to others also! It is just a fun to do, as most of us know everyone in class and we always get a good laugh. Whenever I did it again, I would ask her to slap my hand.
The funny part, is I started doing it at home. I would slap my own hand. Now, it is so automatic, it has been ages (maybe a couple years) that I have left it open.