I know this event has already occurred, but I am making this comment for any future reader that might have this problem and stumble across this thread.
Go to the interview. Do the thing with the very best of your ability, and act like you care. "Why, I already got in somewhere else."
Why? Because the medical community is truly, very very small. For example, I have good friends at a lot of different medical schools who will one day be deans or program directors or just regular ol' clinicians, my current professors have put me into contact with professors at other schools for research, my current Dean was a Residency Director at 3 different places and knows a lot of other Residency Directors, etc. As you get through the system you realize how easy it is to truly track people down.
An example of what you do not want to happen: you decline an interview on the day of or are super rude or extremely offputting or whatever else can make you a really bad candidate -> dean, physician, person of importance remembers this -> goes to different job later down the road -> your application for residency or a job comes back up on this persons table -> probably an immediate. I know that is a stretch, but you pre-meds would be amazed at how truly easy it is to track someone down who knows a guy who knows a guy who has heard something about you in this community.
The point is, don't burn bridges in this profession - especially this early - for things that can easily be controlled.
