I've seen a couple answers that basically say "No is a complete sentence." (Some actually say that word for word.) And while they're technically correct, it's incredibly naive to suggest that on a site where you're trying to work on interpersonal skills.
If someone asks something and you say "No." They are almost certainly not going to just accept it and walk away. They're going to ask again, nag, maybe ask why, etc. You cannot reasonably expect a conversation to end with "No." And even if you could, that's not interpersonal skill building! That is like saying "I found a way to keep my kid away from bad websites - I cut his ethernet cord!"
Good "people people" are able to break bad news in a way that everyone feels like they win, or at the very least those that don't win understand and agree that what you've picked is the correct course of action.
Imagine a question on Workplace.SE where someone says "I've handed out the assignments for my employees by they're not getting it done. What can I do different?" If someone responded "Remind them that you're the boss and that if they don't complete their assignment on time they can be fired." they could expect to get down voted to oblivion.
Also, as a policy it really doesn't work. It reminds me of the drug campaigns of the 80's, and abstenance based sex education, both of which revolve around only saying no, holding your ground, and never justifying your reasons (because when you start, you let someone else pick you apart.) They took the skill out of the conversation and said here's your one line script - stick to it. And statistically both of those efforts were a failure. According to Scientific American, what worked far better than "just say no" was, quote, "the most effective ones involve substantial amounts of interaction."
People don't come to this site because they want to learn how to shut people out. They come to this site to learn how to work with people. Suggesting that someone "just say no" and that they don't need to justify their answer is not working on an interpersonal skill, and we need to put an end to it.