Publisher Lee Goldberg recently related how an aspiring novelist responded when Goldberg sent him a polite rejection of his manuscript. The author wrote:
"Keep printing the same redundant s**t, arrogant ass, just remember the title of this book, u will see it on the best seller list, ***hole."
When Goldberg pointed out this is not the way to win friends in the publishing world, the aspiring novelist apologized for lashing out, but still maintained that his work is brilliant and there's no way that Goldberg could have rejected it so quickly if he'd really read it.
Goldberg added, "It was terrible."
Not only terrible work gets rejected, of course. There are dozens of examples of books and screenplays rejected multiple times before finally achieving publication or production and massive success.
But no matter how convinced you are that the people rejecting your work are clueless and will someday regret their decision, keep it to yourself. The film and publishing worlds both are small and making enemies can catch up with you quickly.
Usually there's no need to reply to a rejection unless the person sending it has given you some constructive feedback. In that case, the proper response is a brief thank-you--even if you don't think it was right. You don't have to pretend you agree, something like 'Thank you for taking the time to give me feedback" is fine.
And don't call them an ***hole.