You have said "I need you to stop" more times than you can count, and it keeps turning into a fight. The problem was never your tone or your timing. Most of what people call a boundary is a demand wearing a boundary's name, and a demand invites exactly the contest you were trying to avoid.
The One-Sentence Test
A single distinction is buried in Fascinating, I'm Sure, MYMOONOVA's book on how four communication styles hear the same sentence in four different ways, and once you see it you cannot unsee it in your own arguments. One version describes what you are going to do. The other describes what someone else has to do. Telling someone you will leave the room if the behavior continues costs you nothing to enforce and asks nothing of anyone. Ordering them to stop hands your outcome to someone else's decision and creates a contest, and contests get entered. A statement about your own next move never does.
The Explanation Is What Starts the War
The line rarely breaks on the stop itself. It breaks on what gets attached to it. A reason offered as courtesy functions as an invitation, and every reason is a surface someone can push against. Offer three reasons and you have handed over three separate arguments to win, and if even one gets won, the whole no reads as negotiable. A stop with nothing attached is uncomfortable to say and close to impossible to argue with, which is exactly why almost nobody uses it.
The Same Sentence Lands Four Different Ways
Say the identical stop to four people who process it differently and you get four separate reactions from one script, which is the whole premise behind MYMOONOVA's DISC profile. A direct, decisive person treats a calm stop as fact and tests it once, briefly, only to confirm it is real. Hold, and the matter is closed for good. A warm, expressive person reads a stop as commentary on the relationship itself unless you say out loud that things between you are okay, so the reassurance matters more than the wording. A steady, patient person stays quiet and remembers everything, and what fades afterward is the goodwill nobody ever had to request in the first place, even while the cooperation continues. A careful, analytical person needs exactly one real reason, and offering three instead of one hands over the weakest one to take apart, dragging the rest of the position down with it.
Say It Once, Not on Loop
Drop the explanation, name your own next move instead of someone else's required one, and match the delivery to how that person processes a stop. That combination is the difference between a boundary that holds in a single sentence and one you find yourself repeating for months. Fascinating, I'm Sure maps this same shift across 98 real situations, one script per communication style. For what happens when the other side turns your stop into exactly the contest you were avoiding, the power game you keep losing covers the mechanics. And if the boundary you need to hold is with family rather than a colleague, the real fight in blended families walks through the version that plays out at a dinner table instead of a meeting room.