The problem is not really that they can get stolen, but remote control, like a bad person gaining access to the car to hit people or something like that.
Not every social problem needs a technical solution. You can steal cars now, this is solved foremost by most people not being thieves and then second the existence of police for the few people that are.
That can be eliminated by not giving direct control and something closer to what Waymo "Remote Assistance Agents" have access to: https://waymo.com/blog/2024/05/fleet-response/