withdrawing application

I have decided where I wanted to attend medical school and I am trying to withraw from one school where I have previously accepted to go and sent in the deposit.
I have sent them a couple of letters saying that I was withdrawing, but I was unable to pull my name off their acceptance list. When I called them they said that they I can only withdraw through the mail. The last letter I sent was certified. I know this sounds really weird. What should I do?
Thanks.
Fritz