Explain your no once. After that, explaining turns into negotiating.

An explanation given once is information, and given twice it becomes a case, and a case can be argued. Each new reason you offer hands over something else to be taken apart, so the conversation quietly becomes about whether your grounds are good enough rather than about your answer. You are not a defendant. The answer was the answer the first time.

Say your reason once, then repeat only the decision: I am not able to. Same words, same tone, however many times it takes. It feels blunt for about five seconds and then it just ends.