I hate people like that >.< I know it was probably easier for you to do that and just get the lady gone, but damnit! Caving in just means they got what they want and know they can just do it again, and get what they want. And yeah, she'll use the "I've done it before!" excuse.
If you can't have a sign on the machine, can you gusy maybe put one up at the customer service desk, or somewhere near the machine?

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