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 correct a workplace rumor about you, use this factual base: state the claim, where it appeared, what can be verified, and who actually needs the correction.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for a correction, removal, or direct conversation and keep an accurate written record. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not publish private evidence, speculate about motive, or recruit an audience to punish someone. Use school, workplace, platform, legal, or police channels for threats, defamation risk, intimate material, or impersonation. You do not need the other person to agree with a boundary for you to follow it consistently.