Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to apologize for mispronouncing someone's name, use this factual base: name the event, role, custom, language, access need, or shared rule and distinguish impact from assumed intent.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for the correct practice or a concrete change and give the relevant person room to explain. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not speak for a whole community, demand education from one person, or use public shame as the first step. Use safeguarding, accessibility, anti-discrimination, security, or emergency routes when rights or safety are at risk. You do not need the other person to agree with a boundary for you to follow it consistently.