Not sure this is a technology-specific problem. You can block a non-self-driving car by simply standing in front of it, too, as long as there is nowhere to reverse to. In both cases, there's not much that the occupant can do without bringing unpleasant consequences onto themselves. What are you going to do? Drive over the person? Do it and you'll be in bigger trouble than the blocker. This is what makes those Just Stop Oil protests do-able: Because a law abiding motorist is largely helpless if a human is in front of their car and won't move.