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! ;-)
You can call them back and press 6 when the greeting starts. That will add you to their "Do not call list" Here are the numbers that called me. .
(951) 366-4459 Nuevo, CA
(859) 203-0846 Warsaw, KY