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 address scope creep with a client, use this factual base: identify the brief, deliverable, rights, fee, revision history, and contract term that applies.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. State what you can deliver next, what approval or payment is required, and when the offer expires. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not release rights or final files beyond the agreement, and do not shame a client publicly. Use platform support, legal advice, or a formal takedown process for fraud, infringement, threats, or identity misuse. You do not need the other person to agree with a boundary for you to follow it consistently.