This sounds harsh, but sometimes we need to do things for the people we love so they get the idea....
Next time your mom gets in the car and takes off call the cops and tell them. they will pull her over. Hopefully she won't continue to drive.
you can also take the keys away, battery out of the car, or disable the car another way.
Dementia was the reason my Great Granny's license was taken away. they finally sold the car so she couldn't drive. which is another option you have.
to be quite honest it sounds like you are enabling her to drive b/c you are afraid to hurt her feelings about it. But what if she physically hurts or kills someone else b/c you didn't want to hurt her feelings?