If you are playing a game, which IDPA is, then it must be the same for everyone. So all use a flashlight or all do not. We have had this happen. One thing you can do is make all targets have black or dark T shirts and then have some of them have a very hard to see no shoot markings. Make them use a flashlight. Move the no shoot T shirts around between shooters. You can force what you want with course design.