You need some way of blocking your drive, obviously.
Was that a wall pillar of some sort on the last picture of your drive? As a short term measure, say until you can find penguin bollards, I'd pop to B&Q, buy some chain and a padlock, and use that to block your drive.
And once you've done that, tell Nicholas his wife's carer can't park on your drive because of all these other random cars taking advantage if you feel his note requires a response. (I missed the explanation for why his wife's carer can't park on his drive)