Like a person already stated, you have to use iBlacklist. What they didn't mention is that you must first jailbreak your iPhone to get Cydia, then install iBlacklist from there. Apple removed it from the App Store, so unfortunately this is a long process.
You can block one number on your iphone. You can download an app called iBlacklist as it will enable you to block the specific number you want. This application is easy to operate and set up. Go to http://www.iphonefaq.org/archives/971686 for details.