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 resolve a recurring household chore conflict, use this factual base: separate the event, money, care, decision, or promise from judgments about loyalty and character.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask each person for their understanding and propose a documented next step or neutral conversation. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not use children, illness, holidays, or relatives as leverage and do not circulate private messages. Use mediation, legal, safeguarding, domestic abuse, or emergency support when power or safety makes direct discussion inappropriate. You do not need the other person to agree with a boundary for you to follow it consistently.