There are no apps and that is not a feature of iOS.
iMessages are handled by Apple's own system, and there is no way to block a specific sender.
SMS messages and voice calls are handled by your cell service provider, so you need to ask them what blocking (if any) features they offer. If the texts are just spam, you should be able to report them to your carrier and they can block that sender. Other blocking options will vary from carrier to carrier, so call yours and ask them what they can do.
You can not block iMessages or calls at this time. This feature has been announced for iOS 7, coming in the fall.
Until such time as it's released, you would have to turn off iMessage and have your carrier block calls and SMS/MMS messages.
If the messages/calls are threatening or harassing, call the police.
It is now possible to report spam or other unwanted messages coming through the iMessages system:
What they can do about it and how soon they'll act I don't know.