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 follow up on a partnership proposal, use this factual base: name the product, proposal, meeting, commercial question, and last agreed next step.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Add one relevant update and ask whether to proceed, defer, change scope, or close the opportunity. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not hide an unsubscribe request, invent scarcity, or escalate around the named decision maker. Follow privacy, consent, procurement, and recordkeeping rules for commercial communication. You do not need the other person to agree with a boundary for you to follow it consistently.