On iOS 7 ?
Go into phone , contacts and choose the contact to block.
Or in recent calls hit the i next to the it and look at the bottom and it says block this caller.
Method 1: Block a Number That Has Recently Sent You an SMS
If someone has recently been sending you harassing or annoying text messages, you can block them directly from the text message app.
Launch the Messages app and select the person you want to block.
Tap on Contact and then Info.
Scroll down and tap Block this Caller.
