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 send a no contact message after harassment, use this factual base: identify the content, account, place, person, channel, access, or behavior that must stop.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Give the required action, any lawful deadline, and the one permitted route for necessary matters. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not debate the boundary repeatedly or threaten consequences you are not entitled to use. Save evidence, secure accounts, reduce access, and contact platform, workplace, legal, or emergency support when risk continues. You do not need the other person to agree with a boundary for you to follow it consistently.