You just don't show up. Then send a thank you letter a few days later saying that you addressed the card in advance but on the day of the interview decided that their program was way beneath you to bother getting out of bed. Then thank them for their interest and wish them luck in their pursuit of further resident education. Or you send a thank you note addressed to one of the faculty members who you know interviews people and comment on the great interview you had and an email to the program coordinator thanking him/her for his/her help and see if they still rank you.
Seriously.....both call and email. I think the old school recommendation was to call and mail, but I think its safe to substitute an email for snail mail, but make sure you get a reply to confirm that they saw it. Do this all as soon as you know you won't go, apologize, and thank them. If you piss them off you could potentially hurt your classmates for years to come.