Those customers must have had enough coffee given that they refused to have another cup of it.
Anyway, there are three ways of offering a drink to someone else, formal (Do you want another cup of coffee?), informal (Would you like another cup?), and very formal (More coffee?), of which you, by intuition, chose probably the most informal one just combining a question and a negation (No more?)!
However, this type of a negative question does exist, and it is called 'negative interrogative sentence'. These 'yes/no' questions usually mean that you expect the answer to be 'yes' rather than 'no'.
So why not to trust your intuition then? ;)