you can only block by a couple of ways
if that person is making threatening calls, you can use a trace by hitting *57 and then make a report and go to court over it.
if that person is making annoying calls you can do the same if they persist after being told once by you, to stop; that's harassment. You can make a police report, use *57 (might be a slight fee per use) and go on from there.
or you can use a service from the phone company called Call Screening (AT&T) other phone companies call it many other names but it lets you put at most, 10 land line numbers on the list and they will hear a message saying youre not taking calls riught now. You may only be able to use this from land line numbers, cell numbers are not blocked with this service; for cell numbers, you go to their cell company or yours, and actually enter the number on your 'block' list; every cell number has the ability to block numbers. it mgiht able to be done off the handset or on the we or with the operator of your cellulr company doing it for you. Ask the company.
Of course you can also let it go to caller ID, disconect your voice mail for a week or two, and tell others (the good ones) to maybe use a different route to contact you,and tell them you'll have voice mail in a few weeks, when poophead stops calling. After getting no voice mail two or three times, i think the calls might stop.