The closest it would come to this is a "kiss-off" letter, short and to the point in most cases. This is a great question because a "Dear John" is usually a woman to a man; I am not sure what the reverse would be or from a man to a man/woman to a woman. Perhaps others have another idea, but that's my take on this, for what it is worth. : )

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Note added at 27 mins (2005-08-21 23:37:35 GMT)
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To be clear, there is no equivalent special term for this male to male other than the one I suggested. However, I am told that there is a "Dear Jane" equivalent for women to men, originating during WWII, apparently. This is something I didn't know. But nothing for man to man, afaik : )

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Note added at 28 mins (2005-08-21 23:38:22 GMT)
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Ooops, correction above, from men to women ;0 "Dear Jane" ...