I'd suggest calling the school and letting the admissions staff know. It's faster and generally they won't ask for a lot of questions. If you really don't want to talk to anyone, they usually have voice mail systems and you could leave a voice mail.
I would be short and sweet. They have too many things to do rather than read a long letter explaining your situation, just keep it short so they know you're not attending and they can offer the next person a position.