You should provide them with something written and ideally signed. It should be something along the lines of:
Dear ____,
I am writing to inform you that I no longer plan on attending XYZ School/College of Pharmacy this fall, as I have recently been accepted to another program. Please feel free to release my spot in the class of 2015 to another candidate. I appreicate your offer of admission and understand that my previously paid deposit remains non-refundable.
Best Regards,
Your Name
Signature
****
A letter is much less awkward than e-mail, because they have to make more of an effort to respond. 🙂