Why do you want to cancel?
I've never actually cancelled an MCAT, so I can't help you. I cancelled an LSAT, which was relatively painless but I got almost nothing for a refund. I'd imagine the same is true for the MCAT. I've you've already spent all that money, it might be worthwhile to write it even though you haven't had enough time to prepare. I wrote last April even though I hadn't studied at all, and I think I learned a lot from the experience that really helped me when I wrote in August.
But, if you really want to cancel, go to <a href="http://www.aamc.org/" target="_blank">http://www.aamc.org/</a> and there should be directions there.