A guy friend of mine begged to differ with my advice to the previous letter writer. He said that he'd prefer the object of his crush to be less opaque about her feelings so that he's not left guessing. But, when pressed, he couldn't come up with what she should actually say. I maintain that there's no polite way to tell someone you're not interested in them, particularly if they haven't expressed interest in YOU.
The burden is on people with a crush to rip the band aid off if they really want to know where they stand with the object of their affection.
However, as I tried to express to "Crushed," I think it's humane....a kindness, a mitzvah...to try to get the point across somehow that she's not interested in dating her pal. Don't make him rip the band aid off if he can avoid it.