The Usenet Oracle has pondered your question deeply.
Your question was:

> O great Oracle, help me out of this predicament:
>
> Being the shy and modest person that I am, I could not get the guts up
> to ask a girl to a prom.  So of course, some desperate girl asks me to
> the prom.  I went with her two years ago and do not particularly desire
> to go with her again.
>
> But, of course, when she asked if I would like to go to the prom with
> her, the word "yes" sprang out of my mouth.  Now what do I do?  I'd
> like to ask someone else, but how do I go about letting the other girl
> down without mentally affecting her for the rest of her life...
>
> Do you have a disappearing spell, or maybe I will be lucky and you have
> a custom made spell just for these sorts of problems?  Help!!
>
> Awaiting your response.

And in response, thus spake the Oracle:

} (What am I, Dear Abby with a copy of the Necronomicon?  Hmnph.)
}
} You actually have TWO problems: you need to get out of your
} committment to take this woman (henceforth referred to as "your girl")
} to the Prom, and you need to salvage your karma.  Currently, for
} leading the poor thing on, you run a dire risk of being reincarnated
} as a woefully unattractive woman with terminal personality deficit.
}
} Here are some possibilities.  Pick the one you prefer and chant its
} title six hundred and sixty-six times, and I'll teleport the
} appropriate grimore to you.
}
} Alter Gender Identity: Turns your girl into a lesbian. She may still
} hold you to the date if she doesn't want to come out of the closet.
}
} Alter Sexual Characteristics: Turns your girl into a fellow. The new
} he probably won't shag you more than a few times before the anger
} dissipates.
}
} Bigby's Lewd but Dextrous Hand: If you can maintain this one for long
} enough, your girl will be too exhausted to go out on Prom night.
}
} Create Zombie: I checked with Miss Manners: zombies cannot (politely)
} hold you to prior social commitments.
}
} Doom of Anne Rice: Causes your girl to become obsessed with vampires.
} Do not cast this one if you are, in fact, a vampire.
}
} Earwigs of Doom:  Effective, but disgusting to watch.
}
} Fandango on Core: Scribbles all over your girl's brain.  Pretty much
} guarantees you'll reincarnate as a computer geek *again*.
}
} Fireball: Zorch the minx.  Of course, you'll reincarnate as a moth in
} a candle factory a few thousand times.
}
} Flight: Take her out to a field, have a confederate fire a magnesium
} flare into the sky, and then fly straight up screaming.  Come back the
} next day and talk to her in a monotone without blinking.  Few women
} want to go out with a guy who's had his brain replaced by aliens.
}
} Grape Ape Rape: Do *not* drop the wand when you cast this one.
}
} Human to Dog: Zap yourself and drool all over her; she should cancel
} the date.  If she doesn't, I want videotapes of the resulting action.
}
} Love Philtre: Drink this in her presence, and you won't mind the prom
} at all.  Makes for some ugly kids later, though.
}
} Pessary of Grue: This spell has only been cast once, on the floor of
} the Imperial Arena during the reign of Caligula.  Never before or
} since have so many people vomited simultaneously.
}
} Quabalah Aquae Mortis: Fills victim's lungs with Manishevitz.
}
} Summon Incubus: Infernally beautiful demon appears and sodomizes
} caster to death.  Extreme, but guaranteed to fix your karma.
}
} You owe the Oracle a spellbook that weighs less than 25 pounds.