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 divide shared furniture after a breakup, use this factual base: list the item, account, lease, pet, vehicle, date, or legal document that still connects you.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer limited options, a neutral exchange method, and a date for closing the practical issue. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not attach relationship arguments to a logistics message or arrive uninvited. Use a third party, public exchange, legal channel, or specialist support when direct contact is unsafe or restricted. You do not need the other person to agree with a boundary for you to follow it consistently.