I second (third?) leaving a note and then getting onto the building management if it needs to be dealt with officially.
A little story about how easy it is to innocently park in the wrong place. A friend of mine used to live in a similar situation with numbered parking, he lived in, say, flat 11. He was going mad because someone kept parking in his spot, number 11, so he had to use the visitor's space, he left notes and complained but got no satisfaction and management kept saying they'd checked the car-park and couldn't see anything wrong. It turned out that the parking spots were numbered differently from the flats and his parking spot was number 3 or something and they guy in 11 had been parking in the right place all along!