Mom has Alzheimer's and a month ago the DMV reissued her a driver's license without a driving test. What can I do?

I agree with Jinx - you must get her off the road. There have been SO MANY posts on this subject. Some other suggestions: take away her keys - disable the ignition switch so the car won't start - have it towed away to the mechanic and let him sell it - tell her it is unrepairable (or will cost more than car is worth to fix it) - call MVC and have them rescind the license since they erred in issuing her a renewal against doctor's orders - take her back to MVC and make them test her - they will automatically fail her, but they will issue her a Disabled Driver ID card that looks like a regular license (she will need it as a form of Govt ID anyway). You will also be able to get the handicap placard to hang in your window when you drive her around. Any way you cut it, your Mom will eventually forget about the car - just make up little stories every time she asks about it.

Tell the police. Tip them off when you know she will be driving. If they come up behind her with a siren, she will drive badly. I actually don't know anything about the legalities, but I bet she would stop driving if a policeman told her to.
Good luck!