How to block a number and let them know that they're blocked (iPhone)?

I would like to block a number on my iPhone and set it up so that every time they try to text me, they get some sort of message that lets them know that they are blocked. Is there any app that allows for this?
If there isn't an app that can do this, I would settle for an app that doesn't block the number but just automatically sends them a message. Is there an app that will send a custom message every time a specific number texts me?

When you block someone the only way they know that they're blocked is if you message them before blocking them.
To block:
Go on the text, on the top right there should be "contact" in blue, tap that and then an i with a circle around it should be one of the options, tap that and then scroll down till you see "block this caller", tap that and they won't be able to call or text you. They'll be able to, but you won't receive the texts and calls.