Pretty strange assignment, if you ask me. Unless somebody is completely out of control or has totally objectionable hygiene or something, why not go through with the date to the earliest reasonable conclusion and then just not make another one? You can still finish the movie, dinner, whatever. No need to shoot someone down completely.
But if you must, there’s also the wild and radical possibility of gentle honesty instead of elaborate lies, which double the insult because unless they are completely credible your date knows you are lying and yet must play along in order not to be a boor back when you are being a boor:
“I’m sorry. I’ve enjoyed meeting you, but I don’t really think this is going to go anywhere. How about if we say good-night now?”