While Apple doesn't offer a program that can block contacts on the iPhone, if you have a jailbroken iPhone you can download an application to accomplish this. Using iBlackList, for example, you can prevent a person from calling and texting you by adding his number to a "blacklist." At some point, you may need to check which contacts you've added to your blacklist. To do this, access the application you downloaded to block your contacts.
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Launch the app you use to block contacts by tapping on its icon on your iPhone.
Tap "Blacklist" or its equivalent -- sometimes called "Blocked Contacts" -- depending on your app.
Tap on the blacklist you want to check if your app offers more than one list. Once you've done this, you'll see a list of all the contacts you've blocked.