I can understand why it is very annoying, however I'm not sure they did anything wrong.
If someones car is plugged in and finished charging and they aren't back, I thought you were allowed to remove it from their car and start your own charge.
It is a bit strange that it is a park & ride at 9pm at night and they've not come back yet for their car, but it's kinda besides the point.
I would suggest this is almost an expected/will become common scenario to find a car plugged in and it has finished charging with no owner around.
At a park & ride I imagine it will happen all of the time, someone's driven their car there and need a charge and are going into town for several hours, they have no choice but to leave the car and know that it'll finished charging before they return.
At my local shopping centre there are several of the pay and display spaces that also have chargers, people will pay for say 2+ hours parking and plug in to charge, they'll be finished charging before they return. Assuming there isn't a charger per space and it's shared between 2 spaces, this situation is going to occur regularly.
The only way around this is for charging stations to have more than 1 ccs cable on them, so that you can use the 2nd cable if someones charge has finished on the other cable. Maybe this will happen.
The chargers should say on them really, "if plugged in and not charging, remove from that car and use". Do any say anything like this on them? I rarely use them.