Unfortunately, apps approved to the App Store aren't able to modify the way the phone works.
Should you decide to jailbreak your phone, there certainly is a tweak that will allow you to do a number of things to callers you want to block. You can have it ring busy when they call, go straight to voicemail, ignore it completely, etc.
Calling your carrier may solve the problem but that sounds to me like it'd be more annoying then putting them in your address book & doing what you already do.