Difficult people need clear lines, not louder voices. Say the boundary once, calmly, and keep it.

Volume signals that a line is negotiable, since a boundary needing shouts has already been tested and found soft. A limit stated once in an even voice and then held without further discussion is much harder to push, because there is nothing to push against. Most escalation is a search for the point where you bend.

Decide the line before the conversation and reduce it to one short sentence. Say it, then stop talking and let the pause be uncomfortable. If it comes up again, repeat the same words rather than producing new arguments.