Retractable barriers have appeared in many an old city center over the last years. The kind that slide into the ground when “allowed” traffic (say, a city bus) needs to pass and come up again afterwards to block all other traffic.
Why not use a similar system for parking bays? The barrier, on the edge of the parking bay would be down until someone parks. Then, the barrier goes up, only going down again after the driver pays the actual parking fee. The advantage is obvious: no more illegal side street parking. Cars will simply not be able to drive away.
Because, similar to existing systems, censors can detect the presence of a car in the bay, the parking bay can send out a signal when someone is parking on the barrier to prevent it from going up.