I have laid down that boudary and i stick by those words i just not sure what yo do to enforce it
How do you stick by those words? Not a trick question or sarcastic, just trying to figure out how you are seeing it.
Instead of thinking of word enforce, try using consequence. She is an adult and you really can't force her to do something, but you can do something that would be a consequence to her for not honoring your boundary. Have you researched the topic, or read the link?
I suggest that in the future you not state a boundary that you have no idea how to carry through with it. You told her you would not live in an open M. Therefore, if she continues with her cheating, it would seem to me that you would take steps to stop living in an open M. Isn't that how you read it? I mean, how can you stick by your words and yet do nothing?
The only effective boundary is one that produces consequences for the one who dishonored it. That is the only way I know how you enforce it.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!