I NEED TO MAKE IMPOSSIBLE FOR MY GRANDPA TO USE HIS CAR.

Hello everyone, Im from Italy and my grandpa ( 84 yo ) will get his license revoked in the near future because his sight and reflex are getting worse really fast. He wasn't bright even when he was yung, but now that he is really old the sitituation has gotten worse. I need to make my grandparent car accessibile only for my grandma ( 73 yo ). Because we fear that he will take the car key and sneak out to go At the bar with his friend. In the past he hitted a pedestrian e he runned away, the guy who he hit wrote his license Plate the next day the Police was at his doorstep. My grandma Who didnt know anything ( my grandpa didn't tell her anything, because he feared that my gm would have "scold" him) almost fainted when she saw Police. This append not once but twice. Luckly the guy decided not to press charges So i need to something to prevent my grandpa sneaking out with the car. I was thinking about a secret kill switch or similar. What do you guys suggest me ? Thanks for help!