You can go to your cell service provider's web site, and they should have a blocking program. Verizion does, anyway. I just log into my Verizon account and enter the number I want to block. Anyone calling after they have been blocked gets a message to the effect that "the number you have called is not accepting calls at this time..." This number just got blocked! ;-)
They just called me and said my car warranty was about to expire and that this is a final notice before closing my file. I didn't answer the call and this is what they left on my voicemail. They didn't provide who they were but indicated they were from the "warranty department". Seems phishy ;)