I Believe In Respecting Other Peoples' Boundaries

You don't set boundaries by asking the other person not to cross a line - that's not a boundary, it's a request - you set boundaries by telling the other person not to cross a line, and making them respect it. With the truly disrespectful, it means telling them that if they keep doing it, you'll walk away, and then immediately walking away when they (inevitably) do it anyway. If they're the kind of toxic person who's incapable of respecting another person's boundaries, then the relationship will disintegrate, because they're simply not a person who you can have both a relationship with and have boundaries with - you can have one or the other, but not both.

But if they're not that kind of person, then setting that kind of hard boundary will finally get it through their thick skull that you're serious about setting boundaries, and that things need to change if they want to keep you in their life.