Usually when I hear a bad idea, I'll ask someone if he or she has done any market validation or talked to any potential customers. If the person says they have not done so, I usually suggest that the person go talk to different people right away. That is my gentle way of telling someone an idea idea isn't too good, as he or she will usually find out after talking to the target market that the original idea wasn't a solution to any problem. That being said, the first time I ever cold-emailed a programmer and Skyped with him asking for feedback on my idea, he told me it was awful and guided me through the process of determining if an idea has "legs". What I learned from him has proved to be invaluable.