How can I block unwanted calls on iPhone 5?
Go to settings, go to do not disturb, turn it on, set who you don't and do want to receive calls from, and go about your day.
Call blocking is not available in prior versions of iOS 7. In iOS6 there is Do Not Distrub option but it will block all your calls and messages. If you have a jailbroken iPhone you can use iBlacklist or just wait until the official version of iOS7 will be released as it will have a blocking option available by default.