This is a great feature on the new ios7 for your iPhone:
blocking callers.

to block someone from phoning you when you have ios7:
open the phone app
go to recent calls
find the number / contact you want to block
tap the (i) info button next to the number / contact
scroll to the bottom of the list and click ‘block’

:D

to unblock them you goto settings > phone > blocked

you will see a list of blocked contacts / numbers