I had a toxic relationship in my life... (really Mary, just one?) and I opened yesterday's post with a situation from it, walking away from the person because they crossed boundaries I had set. With this particular person, I explained why I had taken issue with a subject they continued to broach. Consistently the object of blame and manipulation, I revealed that I could no longer discuss the issue with them. They agreed not to bring it up and was aware that if they did, I would ask them to stop, only once. From there if they persisted, I would leave or ask them to leave, no matter where we were. This conversation about the problem went very well, they agreed to what I said and I came away believing we had an understanding.
Not quite. While the person was rational during the chat, I soon found that they still wielded manipulative behavior and blame like a weapon when they were unhappy. They insisted on not respecting the line I had drawn in the sand. I ended up asking them to leave my house, leaving them at a restaurant and walking away from other numerous gatherings to show them the boundary I set was being disrespected and that was unacceptable. It worked and thank goodness, because this person was in my life for good! They still try and rile me up at times, but I have limited my interaction with them and they are a bit more respectful of my boundaries now.
You can assert some healthy personal boundaries and express those to the other person. For instance I said "when you bring up such and such a topic, it upsets me, so I will ask you not to bring that up." "When you say this particular thing to me, I feel you are doing it to hurt me"or "even if you don't intend to belittle me, what you're saying does." And ask them to stop or refrain from what you feel is negative.