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 invite a neighbor to a community meeting, use this factual base: give the event, date, location, relevant history, and what kind of contact the invitation involves.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a simple way to accept, decline, ask a practical question, or choose a lower-contact option. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not announce their attendance, use relatives to persuade them, or make a public gesture before they agree. Respect no-contact, venue safety, custody, and legal restrictions even when the event feels important. You do not need the other person to agree with a boundary for you to follow it consistently.