I would suggest using Google Voice, which is free and open to anyone now.
You give out your GVoice number, and set GVoice to forward to your cell phone. No one ever needs to know your cell number direct, and if you change it..you just change the setting in GVoice.
GVoice allows you to actually block individual numbers--and generate specific responses to them! And you won't pay for the call, which would use cellular airtime if it was just your cell phone blocking it.
Damned Atrix!Damned Motorola!Damned AT&T!Damned Google!
"Matilda, she take me money and run Venezuela."