Perhaps this is a really simplistic example, but when I sit on my couch, my dog will sometimes try to get her front paws on the couch so that she can get a better scratch behind the ears. As soon as she does, I stop scratching. She then steps back off the couch and I don't start scratching again until she does. Sometimes a lack of positive re-enforcement is enough to enforce a boundary. (Sometimes it isn't.)
It may work for a dog who has no visible alternatives, but when you are dealing with an addicted spouse who has a third party pressuring them to leave the marriage its a bit difficult.
You try getting your dog to cooperate when someone else they like even more is waving cookies at them... Go ahead, I dare you
I don't think that trying to get your spouse not to leave your marriage is a boundary. You can't enforce that. But if your spouse is having an affair, walking away from her is pretty much all that you can do to enforce the fact that you will not participate in the marriage if she is breaking her vows, right? If she screams at you, you don't participate in the conversation. If she misuses marital assets, you deprive her of that opportunity. You can't flog them if they don't do what you want, but you do get to take your ball and leave.
