I arranged for a friend to stay with him temporarily until I could get him into assisted living a couple months later, and she kept the keys hidden and did all the driving. Well, except the time he found the keys and snuck out. Luckily he only drove two blocks to the post office, but boy did I get a hysterical phone call from the friend!
Funny in retrospect, but not at the time.
Once he was in assisted living and didn't have the car parked in front of his house to remind him, he was okay with not driving.