Been using Mr Number for over 8 months now. First on my blackberry, now on my captivate. I HIGHLY recommend. Allows for answer and hangup instead of just forwarding to voicemail as well as SMS blocking. Dont pay AT&T a cent for something that should be freeSearch the market for Mr. Number. It blocks suspected spam, phone numbers by area code, or specific numbers you input. It blocks texts messages also.
Its also free.
Do this then...For phone calls just go into your call log, press and hold the number you want to block. A menu will pop up. Select "add to reject list".
If you don't use one of the above apps on a daily basis and it's installed, whether you like it or not it will interact with the dialer and create a lag.
AT&T has a five dollar per month set up which allows for blocking numbers, etc:
AT&T Smart Controls: AT&T Smart Limits for Wireless
There are also apps in Market which address call and sms blocking.