Free:
Call Firewall 1.4
Pay Software:
Call Block 1.1.7s
I personally use Call Block 1.1.7s, and have switched my voicemail service over to
YouMail, (free). I no longer have a problem with unwanted calls. With Call Firewall, you can block specific numbers, or blocks of numbers, (such as +0* will block all blocked caller IDs, and 8* will block all calls originating from a toll-free area code. You can have it just hang up the call, or pick up the call and hang it up 0.5s later. After switching to Youmail, you can route the messages you don't want to recieve to a junkmail box, and you will never get a notification for it.
CallBlock 1.17s+YouMail is the best combination I have found to rid myself of nuisance callers.