Jailbreak it and download blacklist from the cydia store.
There are many tutorials on YouTube about jailbreak. If you're still in 4.3.3 then just open safari on the iPhone, go to www.jailbreakme.com and follow instructions.
another solution is downloading a ringtone app from AppStore that allows you to record your own sounds, record 30 seconds (or less) of silence and configure this to the contacts you don't like. this way the calls will still come in but you won't hear them anymore.