MCleaner is one of those nifty jailbreak apps which are actually useful. The app blocks unwanted callers (even texts from them), and recently posted an update which adds a feature where the caller gets an immediate answer and hangup - so they can't even leave you a voicemail. Heh.
MCleaner has been around for a while, and runs on quite a few phones (check their site for a full list).
Its available on the iPhone via Cydia (and runs a steep $11.99, actually - up to you if that's something you want to block or accept ) with a 15 day trial.