What’s the best way to take the car keys away from my 90 year old mother. Since I was a child and she gets upset she grabs her keys and goes.

Sorry, but way too little information here for me to answer.
Is your mother competent?
Is your mother able to drive well?
Does mother live with you and have her own car? Alone, and have her own car?

Meanwhile, go to your local DMV or call them and ask this question; be prepared to provide them the needed information, and good luck.

Deactivate the battery. Take the battery out of the car.

Is your Mother a safe driver? In your opinion? What do others that drive with her report?

If she is unafe - be honest & tell her. Then hide the keys.

You can start by disabling her car, i.e. unhooking the battery, or removing a spark plug or two.
Then I would make sure that she has someone that can take her to the places she needs to go, so she has no reason to drive.
And of course you can let her doctor know that she shouldn't be driving and ask them to send a letter to the DMV stating so.
And if all else fails, you will have to take her keys and hide them from her telling her that she must have misplaced them and you don't know where they are.
Please don't allow your mom to drive and put innocent drivers in jeopardy in harms way if you know she shouldn't be on the roadways, as you'll never be able to live with yourself if she injures or God forbid kills someone because of her poor judgement.