I support you in setting a boundary but when she “went off” on you what does that mean, exactly? Here you are not talking about her endless yakking about herself, apparently.
What behaviour will you no longer tolerate from her? What words will you use to express that?
Putting these things in writing is far less effective than face to face conversations and it creates its own kind of problem. If you cannot say the words to her now, you will not be able to enforce the boundary you set.
The important thing to remember is that it is your own behavior that changes when the boundary is crossed. When she does X Bad Thing you stand up to leave or you show her to the door.
Are you ready to do that?
The book everyone recommends is “Difficult Comversations” which lays out some strategies in talking about conflicts.
UL’s little trick is stupid, who has time to play stupid games and what is the point of that, anyway? If you think you are going to teach someone like this a lesson, well, that is not realistic.