How do you deal with people who cannot take no for an answer?

Let me make some side comments about, based in experience.

The NO word is, in most of cases, not enough said by not saying YES. The NO should be clear, firm, not negotiable THERE, where things happen and also necessarily outside your own mind.

When you put your priority in whatever else different to the NO (like avoiding conflict, peace of mind or the "redemption" of that other described people), your NO will NOT work.
That also means that saying "I don´t care of what others think" means "I dont care if the take my ass as long as I look elsewhere". So, own "don´t cares" are just anesthesia for what will anyhow happen.

The NO don´t need to be an explicit ultimatum.
But the message of that some things wouldn´t be tollerated not even twice NEED to me part of the message in some way.
Your NO, to work, in a sense should be inconditional (for you, not negotiable under no foreseable conditions or excuses).
In the other unavoidable sense, conditional: "IF you go on trying, there will be results that you will like FAR LESS than
doing it even one more time".
And prepaired to make it true.

And, finally, if you don´t want to do this cos haves some kind of cost...recognice that (NO MATTER your intentions), your pretended NO is really a YES, an invitation.

Of course, those are just personal opinions, but ones I stand for.

Well, you just don't give in. And you never explain yourself. You don't have to. No means no and make it stick. You don't go back and forth about it. Soon they get the message that you mean what you say. People keep going on and on when they don't think you're stable in your answer or mean it. Show them that you mean it by carrying out what you say you will do.

If it's business i always say....I don't own the house/company/car/phone/laptop...etc.

If it's personal. If they haven't taken the 'subtle hint' say whatever your NO is but say it LOUDLY.
Much harder for people to come off as hard-ons if you publicly embarrass them like that.

I never had any issues getting my point across. 😌

Say no

look them in their eyes and say firmly
I SAID NO!

Deppending on the subject, the person and the context, there ARE ways.

Make it clear no is the only answer they get.

Give them the cold shoulder.

Kick em between the legs? Works for any gender

Thank you for the Best Answer!

I say, "Did I stutter? What did I tell you?"

Walk away let them argue with them self lol

She's a looker that's fact

Not pleasantly

Eventually ignore them.

Disarm them

I stop answering.

According to threat assessment expert Gavin DeBecker, that (unwillingness to accept No) is the most significant sign you have found trouble in the form of a person. You can't change people like that, but you can usually avoid them.