How do I take the car keys from my mother w dementia who wears them around her neck always even sleeping w them?

One method I saw recommended to get people to change their clothes might help here too: have an "oops" moment and spill a glass of something down her front and then help her get into something clean and dry, if it's something really messy she might even need a shower to clean up. Be sure you've got a decoy necklace ready to replace the keys

Disable the car.

Remove the spark plugs from her car.

Remember the club? It is a steering wheel lock. Place it and you hold the key

If you can't get the keys you may need to do something to neutralize the vehicle. Though it does make me nervous that she sleeps with the keys around her neck, that could be a hazard.

Take the battery out of the car.

I agree with disabling the car. We disconnected the battery of my moms van(left the battery in so it wasn’t obvious to her). She was mad because she knew we had done something. Wouldn’t give up her keys for a long time, but made it to where she couldn’t drive either. She ended up throwing her keys at us one day. It was hard & she was really mad, but she was safe. She kept saying she was going to call someone to fix it, but couldn’t figure out who to call.

If she demands to have someone come out to fix it, call someone, explain the situation, & then they come out to look at it or tow it & they’ll need the keys to work on it. Now you have the keys from the repair guy. If it takes more than that,
Either have them tow it some place else or just down the block & give you the keys.

My mom knew we had messed with her van. She was more aware. Depending on the level of decline, less or more theatrics may be needed. Also, the mechanic can just be a friend of yours that she doesn’t recognize.

If the keys have batteries in them, remove and replace with dead ones. Or don't replace at all. Chances are a person with dementia won't remember that the keys have a battery in them and won't understand why car won't start. This worked for me.

In MN, you simply fill out a confidential, one-page form to alert the state of a concern about an individual's driving capabilities. We did this for both of my in-laws, and they received a letter with an appointed time to meet with someone at the DMV. Another family member accompanied them and they both voluntarily surrendered their DLs after some back and forth -- the other option was to take a new driving test. They went through the process of getting state ID cards right then, before leaving the DMV.

Hopefully other states have a similar setup: simple, helpful, and takes most of the responsibility off of the family and places it (appropriately) with the agency responsible for licensing oversight. 

2011 in October:I took my mother for her driving examination when she was 91, almost 92, and she failed her exam when she drove her vehicle into a curb at DMV instead of pulling out straight. Her license actually got suspended back in 2008 with a heart condition but still kept driving. In 2012, I took over when I was unemployed and did this for over one year in 2013. I took her car to a mechanic for its inspection and got her $500 estimate to fix a broken steering wheel part. Mom did not pay the expense, so she sold the car and never drove again!!

if your mother will not stop driving, disable the vehicle like so many readers here said.