Be calm. Joke about it. Make light of it. But make it clear that you're doing the intoxicated person a favor.
Locate their keys while they're preoccupied and take them away. They will probably think they've lost them and will be forced to accept another mode of transportation.
Try to use a soft, calm approach. Suggest to them privately that they've had too much to drink and it would be better if someone else drove them home or if they took a cab or other transportation.
Tell them that if they insist on driving, you are not going with them. Tell them that you will ride with someone else, take public transportation, or walk. Don’t put yourself in harm’s way because they won’t listen.
If it's someone you don't know well, speak to their friends and have them make an attempt to persuade them to hand over the keys.
Avoid embarrassing the person or being confrontational.