Austin Kleon has assembled a bunch of "no thank you" form letters from writers and publishers. I've seen a few of these before. My favorite is Edmund Wilson's postcard, where he says, "it is impossible for him under any circumstances to receive unknown persons who have no apparent business with him."
A couple of years ago, I was getting sent this article, "Creative People Say No," at least twice a day. The idea was that creative geniuses say "no" to a lot of requests (like, a psychology professor researching processes of creative genius) in order to get their work done, so if you want to be a creative genius, you have to say no a lot so you can get your work done.
A bunch of people asked me what I thought about it, and I said, "It's good advice for the rich and famous. Creative people say yes until they have enough work that they can say no."