Toxic people are manipulative and they don't like to be caught or be called out for it. The world needs assertive people like you to hold them accountable. Because I am working on assertiveness, I admire this quality of yours.

But if you don't want to deal with them on a long-term basis, it's best not to go back and forth with them. If you were assertive with them once, that's enough. Next time they say something, you can say, "We've already discussed this and there is nothing further to discuss." Yes, they are relentless, but what they want is for you to fuel their fire. So you have to be patient and consistent in saying, "We've already discussed this and there is nothing further to discuss." Their fire will eventually burn out or they will go find other sources of fuel.