Instead of saying "no" to their idea, try to say "yes" to something else. Ask yourself, what could they do that would prove it's a good idea? For example, maybe

Instead of saying "no" to their idea, try to say "yes" to something else. Ask yourself, what could they do that would prove it's a good idea? For example, maybe they could line up a customer who would pay for it, or an investors would would pay to have it developed. Then you can suggest they take this additional action.