I think the Klingon approach would be to challenge the requester to buy something or die (though it was never clear to me if you were then supposed to kill them

I think the Klingon approach would be to challenge the requester to buy something or die (though it was never clear to me if you were then supposed to kill them or if they were expected to perform hari-kari in lieu of purchasing something - both seem like a lot of extra work and overhead) But if your not Klingon then I think a simple “No” would do. You could also follow the example of my local Dunkin’ Donuts and A) stare at the requester in silence until they go away B) pretend every question is a grand mystery to be solved and start looking for clues on the surrounding walls, ceiling, and floor C) respond with unrelated nonsense questions like “white wall or black”, “red sauce or marinara”, “does the hulk use sunscreen”, etc.