Clarify the agreement that existed
Identify the promise, policy, schedule, ownership, or shared understanding that applies before assigning blame. For a message about how to decline a baby shower invitation, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Resolve the next dependency
State what is complete, what remains disputed, and which action would move the situation forward. 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.
Document without threatening
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. Keep a factual record and describe only remedies you are entitled and prepared to use.