How do I get my 84-year-old father to stop driving when he is putting everyone in danger even though he thinks he is a safe driver?

Enlist the aid of someone else - a "third party." (perhaps a doctor, minister, friend, relative.) Have a frank talk with him about the dangers of driving beyond his skill level now. Sometimes when they hear another voice chiming in it is easier.
I know it will be hard for me to give up my keys. An elderly friend of ours said it felt like a kind of "death" for her to stop driving. It will take some adjustment. But, bottom line is you have to get those keys away from him before he injures himself or worse yet takes a life.

You have to stop him whatever it takes if you feel he is unsafe to others-disable his car or just tell him you will report him to the police and set up a way he can get out and about it may be hard for the family at first-check with your senior center to see if they have a bus to take him to activities and MD visits maybe the MVD will have him take a road test-if he is found lost or confused they will do it anyway or have him take a driving lesson and they will see the problem -how would you feel if his actions caused someone harm.