Driving - how can I stop my Dad from driving?

Report them to your local DMV as seniors at risk who will not listen to you. Ask that their licenses be suspended until they complete driving exam, written test and vision test. If you are not their guardian there is little else you can do. My brother "knew something was wrong" but he drove until he had a very serious accident. Luckily no one else was hurt, but he lay bleeding in the lap of a neighbor saying "I knew something was wrong; I knew something was wrong". I hope you won't have to go through that, but sadly it happens more than you know.

Please, please do not let him have the car or resume driving in any other car!! Myasthenia Gravis?? Aren't there laws in his state requiring his physician to report certain medical conditions to the Dept. of Motor Vehicle so the license can be suspended/revoked? If not there should be.

Been there done that, but I'm wondering: have you watched him drive? Or driven with him as a passenger or followed behind him in your own car? How far do they go? Is your father in danger of forgetting where he was going or how to get home?

My own mother is 91 and has a tiny bit of short-term memory loss, usually mostly manifests at the end of the day (Sundowning). She drives to the grocery and department stores all within a 3-mile distance. She only goes when there is no bad weather and the streets are dry. She only goes out between rush hour/school busses. She has a cell phone on her at all times. Occasionally I will follow her (as secretly as I can) or have her drive me somewhere just to make sure she's ok. She is very healthy and has good vision, hearing, and motion in her neck and back (so she can turn to look). Most importantly she still has good judgement. She knows "someday" I will help her close the chapter on this activity, as I've done for 3 other seniors who were no safe to drive.

My long point is: just because your father is 91 doesn't mean it's "time". Maybe it is... but Instead of battling him with an abrupt end, arrange for their family, friends, neighbors, church acquaitances to take them to appointments, run errands, take a casual scenic drive. Give the volunteers a gc to a favorite restaurant so they take your parents out to lunch or dinner. The social aspect will have far more benefit and he will get used to not driving without realizing it. I did this for my aunts in FL.
It worked great.

In the meantime if he truly is unsafe to himself or others on the road, report him anonymously to his state's DMV (online). They will send him a letter to come in to get reexamined (eye test, maybe behind the wheel). Make sure no one takes him to this appointment and let his license expire. Or, intersect his mail so he can't act upon the letter. When my Aunt got hers she asked my cousin to drive her to the DMV so she could renew ;-) That didn't happen. I just made sure they had scheduled rides to anywhere. The only difference is there is not more "spontaneous" driving. And that's ok. Covid has helped limit some of this.

If he's very stubborn and unsafe, you will need to remove not just his keys, but his car. Make up a therapeutic fib ("the car's in the shop needing a very costly repair"). Park the car where he can't locate it or report it as stolen. Make sure all those he might contact know to NOT lend him their car.

To be fair, it's not just the loss of the freedom, it's what the end of driving (and other activities) means: one more step closer to the end of his very full life. I wish you much success in navigating this with your parents and peace in your heart.