I am friends with a diagnosed narcissist. In fact, I would say he is my best friend. I have known him for almost 20 years now, and he taught me how to say 'no' effectively. He didn't intentionally do so, but he did; he taught me by not taking 'no' as an answer.
It used to be like this:
Him: Come out to dinner. (or other activity)
Me: I can't, I'm trying to budget.
Him: I'll shout.
Me: And I'm tired, so I can't.
Him: You'll feel better after food.
Me: Also my calendar is very busy and I was looking forward to a night off.
Him: Pfft, come out to dinner anyway.
Me: But *excuse/reason*
Him: *knocks excuse/reason over*
*Repeat until I give in and he gets avoid eating out alone (or other activity)*
So many times, so many years. I found myself giving in to what he wanted so many times before I finally learnt how to set effective boundaries.
This is what I learnt. I learnt to not give a reason. The first few times I tried to not give a reason, it was very hard. It went against my grain to not give a reason. Often, when met with repeated "But why? Just tell me why" over and over, I'd end up giving a reason, he would knock the reason flat, and I would end up yet again doing whatever he wanted. My friend the narcissist is a debater and manipulator and has honed those skills to an art. If I gave a reason for saying 'no', he would debate against it and win. He wanted what he wanted and my 'no' was just a hurdle to getting what he wanted. It was a challenge to be eliminated, and my reason was the chink in my armour that he could attack. Eventually, I learnt to answer repeats of "But why?" with "You just need to accept my 'no', I don't have to tell you why" and repeated that back a few times. Eventually, after having the same conversation many times, he learnt too. He learnt that my 'no' really did mean 'no'. Generally, I would still quite often acquiesce with some encouragement, but if I stood my ground firmly, then he didn't bother arguing for very long because he learnt it was pointless. This new pattern has persisted since then, and I'm a lot happier for it.
Personally, I like getting reasons. I can accept things much easier if I just know WHY that thing is so. It drives me nuts not knowing why. Also, I figure that a reasonable person will hear a reason and be reasonable; I much prefer giving and getting reasons. Giving reasons doesn't work with everyone though.
Here's what giving a reason potentially does:
1) The other person accepts that you have your reasons, and moves on.
2) The other person judges the reason by their own yardstick instead of yours. They may decide you are making excuses instead of having valid reasons. They may then argue against it, or decide you aren't putting any effort into the friendship, or dismiss your legitimate concerns.
3) The other person doesn't care about any of your reasons and just uses it as something to argue against to get what they want.
A simple "oh I'm so sorry but I just can't today. Next time!" works much better in most cases. It's polite but offers no extra thing to judge, it offers no item to argue against, and a person who was likely to accept a reason is just as likely to accept a no without one. Since discovering the No Reasons principle, I have extended it into other areas of my life too. Inevitably, when I find myself kicking myself later because an interaction went badly, I'll think about it and find that it all start going wrong when I gave a reason.
Be polite. Be concise. Be firm. Give No Reason. They'll eventually learn to take your 'no' as an answer.
As a sidenote, if someone REALLY "won't take no for an answer", then they aren't actually asking a question any more. They are trying to demand, command, and control. Learn to shut that shit DOWN. An answer requires a genuine question to be asked first, a fake question does not need to be respected.
thalio says: Similar idea: "Remember that in giving any reason at all for refusing, you lay some foundation for a future request." - Sir Arthur Helps