Unless your jail broken currently you cannot.
When iOS 7 is released in a few months time their should be a feature that allows you to block numbers then.
It's all being Beta Tested as we speak.
If someone is bothering you then their are steps such as 02 changing your number free once or report the caller to the authorities.
You could resister here
http://www.tpsonline.org.uk/tps/mobile/index.html
