Can you be more specific? Are you trying to block a person from calling you or are you trying to prevent being able to make a call to a certain number? Call Intercept is a service that you need to subscribe to, and it has this feature:
You are able to select up to 25 telephone numbers that Call Intercept will block from reaching you. You can add up to 15 telephone numbers, which will ring through as Priority Caller.
I'm a little confused on the Call Intercept feature. If it blocks anonymous, private, etc. calls, along with the numbers on the blocked list, why would a priority list be needed? Wouldn't the calls from people you know come through already and not need to be on the priority list?