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.
Launch the app you use to block contacts by tapping on its icon on your iPhone.
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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.
If you remove the blocking application from your iPhone, all the blocked contacts are unblocked.