based on my experience with Verizon, most carriers don't allow you to block numbers anymore...unless you're willing to pay for that service! i know Verizon charged, but can't speak for AT&T. hopefully iPhone will have an app for that

free of charge. but for now, there is this program:
iBlacklist for iPhone - Call filter - Call Block - Sms Mms Filter. the free version offers 2 blocked #'s only. but the paid version ($12 i think) allows more.
there are other ways of dealing with it too. like adding the number as a contact and setting the ringtone to silent (i think you can do individual ringtones on iPhone, right??). or (last resort) change your number.
good luck