I've run into this problem a couple of times. People have come to me asking for commissions and requesting I do the art in a different style. -_o Most recently it was someone asking me to draw their Jack Russle character and then they showed me a drawing done by Blotch and instructed me to make it look like that! In the past I have tried to calmly explain that I was NOT "such-and-such" artist and that if they wanted a piece on in a certain artist's style, then they should commission THAT artist.
Though sometimes the message just doesn't seem to go through and once the piece is done, I'll get asked again why it doesn't look like something "that other artist" would have done.
Does anyone else have any advice for dealing with this sort of problem?