Eightbore, I looked in my ISSF Rulebook and T 9 65 7 says the shooter may refuse the shot if the target is not released immediately after the shooter's call. Now you have me curious, because I was discussing Pia Hansen's antics with Lloyd Woodhouse while watching her bounce pellets off the top of the bunker, Lloyd said putting a slight delay in would cure that, I thought they did.
I'll ask USA Shooting about that. Lloyd has retired, of course.