If you are running iOS 7 (the latest OS, only compatible with iPhone 4 or newer), there is an internal option to block numbers. When you receive a call from a number you want to block, after hanging up:
1) Open the "Phone" app.
2) Tap the "Recents" tab at the bottom
3) Tap on the (i) icon
4) Scroll to the bottom and tap on "Block this Caller"
This should block them for phone calls, texts and FaceTime requests.