"Do you like me? Check yes or no. P.S. I don't like you."
Most notable (and most unnecessary, considering that anyone receiving such a letter would fuck straight off anyway), was "Please do not write to me again."
Heinlein's contemporary in both science fiction and dickotry, Harlan Ellison, took a different tack: He answered his fans' messages in excruciating, brain-bursting detail, all while making said fans feel as shitty as humanly possible for forcing him to do so.
"P.S. I'm dead. I'm now dead. And it's all your letter's fault."
Ellison's 1989 response to the very first letter received by one Thomas Pluck -- not just a fan, but a card-carrying super fan -- begins thusly:
"All a writer has is time and a portion of talent. Answering queries from readers eats away at the former, thus disallowing full use of the latter. I continue to beseech my readers not to burden me with this sort of personal need, but every day I receive a dozen items that demand my response. Yours is one of them. My wife advises me that you are a HERC member, and thus are deserving of attention, but I cannot conceal my annoyance at having to depart from deadline work to satisfy your curiosity. Please don't do this to me again."
He then goes on to painstakingly address the inquiry before signing off with the particularly sardonic "All best otherwise." Mr. Pluck presumably re-upped his fan club membership to the lifetime option immediately after receiving this glorious response.