One flat for each parking space he/she took up!
That's terrible that someone flattened the tires. I can see how that would annoy some people, but it's no cause for damaging the car. Confront the owner, but that would mean face-to-face conflict and most people are cowards when it comes to that. Perfect example is how people speak to others on social media. They wouldn't say what they posted to someone's face. Cowards...
I know that people who do this are just trying to protect their paint from chips, but they should just park farther away from the store entrance and they wouldn't need to take up two spots, but most are too lazy and selfish.
I always park way out of the way. Heck, I could use the exercise anyway!