If you jailbreak there is an app called iBlacklist. There is an app in the appstore with the same name - but it is NOT the same. The one in the appstore does nothing, but the one you can use after jailbreaking works fantastic! I can't recommend it enough.
Without Jailbreaking - I have never found a solution in about 1 year of searching. iBlacklist was the main reason I jailbroke my phone in the first place. Of course, since then I found countless advantages to jailbreaking and (personally) never found a single reason to not jailbreak.
Take care,
Rob