iBlacklist is an app that lets you create blacklists on your phone so that you can block calls and messages from people you don't want to hear from. This is great for getting rid of those people that found your number and just won't leave you alone. 

What I love about iBlacklist is that it features both blacklists and whitelists so that you can choose how you protect your phone. Furthermore, it allows you to separate between MMS, SMS, and phone calls in the event you want to let individual messages pass.

You can also import to your whitelist and blacklist from your history, which makes it super easy to block numbers in a snap. But perhaps my favorite feature is the ability to auto-reply to messages from blocked callers, which gives you the chance for the last word, if you want it.

Overally, I really dig this app, and it's one of the few that I recommend in this price bracket.

iBlacklist has a free trial so that you can get a chance to try it out before you pay, which should give you just enough time to try out the dozens of different settings it offers.