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 reconnect with a past investor, use this factual base: remind the person where you worked together, when, and what current topic makes the contact relevant.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Make one modest request and give enough information for them to decline or redirect it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not imply endorsement, exploit a former hierarchy, or send confidential material from the old relationship. Use official business, privacy, conflict, and procurement rules when commercial or regulated information is involved. You do not need the other person to agree with a boundary for you to follow it consistently.