Start with the part that is verifiable
Separate the event you can describe from the motive or reaction you are guessing. For a message about how to decline a request for unpaid work, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Make one answerable request
Give the recipient one concrete question or next step instead of several emotional jobs at once. Give a direct no, a limited alternative only if you mean it, and any practical information they still need. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Leave the recipient in control
Do not overexplain private details, blame another person, or say maybe when the answer is no. When pressure, coercion, or retaliation is possible, use a safer channel and involve appropriate support. A clear message remains useful even when the answer is no or no answer arrives.