How to block a number on a iPhone?

I've downloaded apps but none of the ones I've found worked.

Apps won't do anything you have to either access your account online or go into the store. Either way you you need to know the password to the account and tell them the number you want to block. It's easier online.

It is not possible. I'm a developer for Apple and am running iOS 7 on my iPhone. It comes with an option to block numbers so you are not bothered when they try and contact you. I guess you will have to wait until October when it comes out, or google how to install iOS 7 on your device and start blocking it as of now.

I'm running the iOS 7 beta and yes you can block once iOS 7 is released publicly in the fall.