I find that blocking numbers is often more of a hassle to do than it’s worth because the iPhone doesn’t have that option built-in. And, according to a few Verizon techs, the iPhone is not compatible with certain other features of the Verizon network, like the Call&Message Block that @AshlynM mentions either. Yet another reason I went Android; I like features. That Apple logo severely limits your features and options.
There is a $1.99 app called Blacklist which is like the iBlacklist that @AshlynM recommends except that you don’t need to jailbreak your phone. The catch is that it’s crap; like many other apps, it is pure snake oil. If you want an app that actually does block numbers, you will need to jailbreak your phone with all of the risks and consequences that entails.
But there is also the “Silent ringtone”, which seems to be the most common method as it is easy, free, and doesn’t void your warranty. This is likely your best option.
Personally, most of the people I want blocked are not the type to leave messages, so I set my Droid X to kick those numbers directly to voicemail, secure in the knowledge that they won’t fill my inbox.