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 decline personal questions from a coworker, use this factual base: describe the request or behavior, its frequency, and the effect on delivery, health, or privacy.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Propose a workable channel, schedule, scope, or escalation route and document the agreement. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not retaliate, shame a colleague, or refuse essential duties without explaining the operational alternative. Use HR, a representative, occupational health, security, or legal support for harassment, discrimination, threats, or unsafe work. You do not need the other person to agree with a boundary for you to follow it consistently.