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 thank a stranger who returned a lost item, use this factual base: name what the person did, when it mattered, and the practical or emotional difference it made.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Thank them directly and share one honest outcome they helped make possible. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not exaggerate, create a public spotlight without consent, or attach a new favor to the same message. Protect private health, financial, workplace, and family information when describing their help. You do not need the other person to agree with a boundary for you to follow it consistently.