Having off-limit topics is a way of micro-managing relationships. I would suggest that instead, you have a ready response to things that you don’t want to discuss with her. Saying she can’t say things is trying to manage her. But saying your ready response when she broaches those topics you don’t want to discuss, would be a way of controlling yourself rather than her. For example, you could say, “Mom, this is something that I don’t want to discuss with you because we always disagree. I will walk away if you continue to talk about this.” might be more effective and keep you working on your side of the street rather than trying to manage her side (ie: trying to set up a fence around what she can and cannot say to you ahead of time).