If the "Do Not Fly" list is a good idea, it probably makes sense to apply it to car rentals if they don't already.
Terrorists will find a way to terrorize if they want to. The world is so chaotic and leaky that you can't stop people from doing stuff like this.
It isn't.
Anything less is simply amateur hour counter-terrorism technique, and a massive civil liberties violation to boot.
Let's not forget that not so long ago, terrorists were doing attacks with public vehicles.
They have time to plan and prepare, so they'll always chose a cost-effective way. Block one, and they'll pick another.