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 end a mentoring relationship respectfully, use this factual base: state the role, agreement, effective date, unfinished work, property, access, and any transition obligation.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Give a clear ending, complete the required handoff, and name what support you can realistically provide. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not disappear, promise indefinite availability, or use the ending to relitigate every grievance. Follow contracts, safeguarding, data access, employment, custody, and legal requirements before the role ends. You do not need the other person to agree with a boundary for you to follow it consistently.