Those are no-win situations, and there is really nothing you can or should tell them. You have that 1-5% element that may even sneak past your phone interview or look good on paper! but that you almost immediately know will never work for you. I don't think lying is ever a great idea, but in those situations the truth is a very risky endeavor, and some sort of white lie ("we just filled the position") could be an option.
