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 tell customers your business is closing, use this factual base: lead with the confirmed event, what it changes, who already knows, and any practical next step.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Say what support or decision is needed now and make clear when no response is required. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not hide the central fact behind a long preface, speculate, or offer reassurance you cannot support. For death, abuse, medical emergencies, legal danger, or immediate safety risk, use qualified and urgent support first. You do not need the other person to agree with a boundary for you to follow it consistently.