You would actually want event 18 to run if you had more than instance or they wouldn't move. You only want to prevent it running if you have 0 instances.
I was just looking at it again and I see you're not destroying the staples that hit boxes. Then next time you click you create a new staple and then the 2 of them are positioned at the stick man's hand and fired together. If you keep shooting at a box you're firing more and more staples every time.