Use this three-sentence script whenever you must refuse a request:
Sentence 1: State the risk. 'We cannot do X because it creates Y risk.'
Sentence 2: Give the reason with evidence. 'The reason is Z, based on the test report / capacity plan / compliance record.'
Sentence 3: Offer an alternative. 'What we can do instead is A, which protects B and still meets C.'
Keep your tone calm, your evidence ready, and your alternative specific.