To continue my tangent from last week:
Also in this article on Agent Query that I read, they mentioned that one of the reasons agents don't respond to email queries is because they don't want to invite dialogue. This I completely understand. If Agent A sends Author B a rejection, and Author B thinks Agent A should reconsider, what is stopping them from hitting Reply? If it's a letter, it's more difficult, though not impossible. Why do authors do this? It's drives me crazy! Take the "No," shrug your shoulders, and send it somewhere else. After all, it's not a personal attack on you, it's a business decision. Quit wasting the agent's time with your piddly B.S. If they wanted it, they would have told you that! You arguing with them isn't going to make them want to read it; it's going to make them black-ball you.
On the other hand, I don't think this should stop agents from sending some kind of response to let the author know they received the query. So one silly author sends you a reply and demands that you look at his fabulous work, so delete that one. If he keeps sending you stuff, block him as a sender. It can be that easy.