IF AT&T can't block those number for you, then there is no other way to do so. No app or feature on the iphone itself can block numbers.
You may have to wait until IOS 7 comes out there might be a feature to dp that http://www.apple.com/ios/ios7/
Appears IOS7 will block specific numbers now, BUT it still won't block calls that do not have a Caller ID.
I contacted apple and they suggested I submit the feature as feedback: