How do I block unwanted callers without jailbreaking?
A new feature of iOS 7 is call blocking and it allows the device to block calls apart from the previous solutions that involved jailbreaking or having the carrier block calls for you.
Go to recent calls within the phone app. Tap on the "i" to the far right of the number you wish to block. On this info page, scroll down. At the bottom of the page, tap "Block this Caller". Confirm on the warning message.
From the settings app, use iMessage and FaceTime preferences to manage the block list.
This list blocks calls, messages and FaceTime calls (audio and video) from that contact. Combined with a blocked contact as Loïc Wolff has proposed, you have a neat solution for dispatching unwanted calls on an ongoing basis.
Before iOS 7, the only way to block callers is to create a "Don't Answer" contact and assign the numbers you want blocked to it.
That way, you'll know not to answer when those people call. You can also assign the contact a "silent" ringtone.
If the number is still in your "recent calls" list, simply click on the circled "i" on the right of the number. Scroll down to "Block this Caller" and then activate it That simple.
In iOS 7 caller blocking is supported by the OS. Unless I'm missing something here the number you want to block needs to be in your contact list. So to block a new number you still need to add a contact first which seems a bit backwards.