there is a program called iblacklist, this lets u create groups of people (in ur case u can put everyone in) and create auto text message replies if an SMS is received from them. It has a whole host of other uses but its the only app that i know that can do what u want. U just have to enable it when u want the auto reply to be sent and disable it after, im fairly sure this will also block calls tho, if thats something ud want to do
Hope this helps