Automatic SMS Reply With Repli
Repli is a free Windows Phone application for those that would like to automatically respond to received text messages when busy or in a meeting, much like an out of the office auto responder.
Install to your Device and start it. Customize the text at the top to the one of your liking and that text will be automatically sent as a reply to any incoming text message until the Stop button is pressed or the application is closed, the program also keeping track of the Contact and numbers it has auto-replied to.
(Source: XDA Developers)