If “No one likes a tattletale, Marjorie” isn’t professional enough for you, maybe try “I’m growing increasingly concerned about how much time you’re spending tracking your coworkers’ breaks when you have your own work to do, especially since I’ve already told you that this isn’t your responsibility. Going forward, I need you to focus on your own performance. I hope we don’t need to have this discussion again.”

Of course, no matter what you do, there’s a chance that Marjorie’s going to go over your head with this eventually, but it sounds like you’re prepared for that situation.